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Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Bacterial Pathogens of Humans

(This chapter has 6 pages)

© 2008 Kenneth Todar, PhD

Historically, bacteria have been the cause of some of the most deadly diseases and widespread epidemics of human civilization. Smallpox and malaria, diseases caused by other microbes, have killed more humans than bacterial diseases, but diseases such as tuberculosis, typhus, plague, diphtheria, typhoid, cholera, dysentery and pneumonia have taken a large toll of humanity. At the beginning of the Twentieth Century, pneumonia, tuberculosis and diarrhea were the three leading causes of death (Figure 1). Water purification, immunization (vaccination) and antibiotic treatment have reduced the morbidity and the mortality of bacterial disease in the Twenty-first Century, at least in the developed world where these are acceptable cultural practices (Figures 2 and 3).

Figure 1. CDC.


Figure 2. CDC.


Figure 3. CDC.

Albeit, some bacterial diseases have been conquered (for the present), but many new bacterial pathogens have been recognized in the past 30 years, and many "old" bacterial pathogens, such as Staphylococcus aureus and Mycobacterium tuberculosis, have emerged with new forms of virulence and new patterns of resistance to antimicrobial agents (Table 1). Great vigilance is warranted, and research and study are needed to control both old and new bacterial pathogens.

Table 1. Examples of bacterial pathogens and diseases recognized or reemerged since 1977
Bacterium Disease
Legionella pneumophila Legionnaires' pneumonia
Listeria monocytogenes listeriosis
Campylobacter jejuni gastroenteritis distributed world-wide
Staphylococcus aureus toxic shock syndrome
E. coli O157:H7 hemorrhagic colitis; hemolytic uremic syndrome
Borrelia burgdorferi Lyme Disease and complications
Helicobacter pylori gastric and duodenal ulcers
Ehrlichia chaffeensis human ehrlichiosis
Clostridium difficile antibiotic induced diarrhea; pseudomembranous colitis
Vibrio cholerae O139 epidemic cholera
Salmonella enterica Serotype Typhimurium DT 104 salmonellosis
Bartonella henselae cat scratch fever
Streptococcus pyogenes (Group A Strep)
necrotizing fasciitis (GAS); streptococcal toxic shock syndrome
Multiple drug resistant S. aureus (e.g. MRSA)
nosocomial and community associated infections
Chlamydia pneumoniae atherosclerosis
Clostridium botulinum sudden infant death syndrome (SIDS)
Vibrio vulnificus wound infection, septicemia, gastrointestinal disease
Parachlamydia pneumonia
Corynebacterium amycolatum hospital-acquired endocarditis
Klebsiella pneumoniae
blood stream infections
Linezolid-resistant enterococci (E. faecalis and E. faecium)
nosocomial infections
Multiple drug resistant Acinetobacter baumannii nosocomial infections



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The Major Groups of Bacterial Pathogens

This article deals with the major groups of bacterial pathogens. Specific chapters in the textbook address many individual groups or members of a group. All groups are defined by at least one bacteriological criterion such as Gram stain, metabolism, morphology, spore formation, etc. However, there is often some genetic or phylogenetic relationship between members of a group. Although we organize bacterial pathogens into natural groups for discussion based on bacteriological criteria, rather than on the basis of affected organ, mode of transmission, or type of disease, two summary tables are provided at the end of this reading that identify bacterial pathogens of humans on the basis of specific bacterium, type of disease, and usual mode of transmission.

When one searches for clusters of pathogens in the Bacterial Domain of the Tree of Life, they are found primarily among the Gram-positive bacteria and the Gram-negative proteobacteria. Most of the bacterial pathogens of humans are classified as Gram-positive or Gram-negative, but some notable exceptions include the mycoplasmas, chlamydiae, spirochetes and the mycobacteria. 

Spirochetes

The spirochetes are a phylogenetically distinct group of bacteria which have a unique cell morphology and mode of motility. Spirochetes are very thin, flexible, spiral-shaped bacteria that move by means of structures called axial filaments or endoflagella. The flagellar filaments are contained within a sheath between the cell wall peptidoglycan and an outer membrane. The filaments flex or rotate within their sheath, which causes the cells to bend, flex and rotate during movement. Most spirochetes are free living (in muds and sediments), or live in associations with animals (e.g. in the oral cavity or GI tract). A few are pathogens of animals, occasionally transmitted to humans (e.g. leptospirosis). The two major pathogens of humans are Treponema pallidum, the agent of syphilis, a sexually transmitted disease, and Borrelia burgdorferi, cause of Lyme Disease, transmitted by the bite of the deer tick.

Figure 4. Spirochetes: A. Cross section of a spirochete showing the location of endoflagella between the inner membrane and outer sheath; B. Borrelia burgdorferi, the agent of Lyme disease; C. Treponema pallidum, the spirochete that causes syphilis. (CDC)

Spirilla and other curved bacteria

Spirilla are Gram-negative bacteria with a helical or spiral shape. Their metabolism is respiratory and never fermentative. Unlike spirochetes, they have a rigid cell wall and are motile by means of ordinary polar flagella. Two important pathogens of humans are found among the spiral forms. Campylobacter jejuni is the cause of bacterial diarrhea, especially in children. The bacterium is transmitted via contaminated food, usually undercooked poultry or shellfish, or untreated drinking water. Helicobacter pylori is able to colonize the gastric mucosal cells of humans, i.e., the lining of the stomach, and it has been well established as the cause of peptic ulcers and there is strong evidence for its involvement in adenocarcinoma.


Figure 5. Helicobacter pylori

Vibrios

The term vibrio refers to a Gram-negative bacterium which has the cell shape of a curved rod or a comma. Members of the genus Vibrio consists of common bacteria in aquatic environments, especially marine environments.  They have structural and metabolic properties that overlap with both the enterics and the pseudomonads. Vibrios are facultative anaerobes (grow in the presence or absence of O2), like enterics, but they have polar flagella, are oxidase-positive, and degrade sugars in the same manner as the pseudomonads. In aquatic habitats, they overlap with the pseudomonads in their ecology, although pseudomonads favor fresh water and vibrios prefer salt water. Some marine vibrios are bioluminescent (they emit light) and some are symbionts of fish, squid and other marine life.  Vibrio cholerae causes  epidemic or Asiatic cholera which, untreated, is one of the most rapidly fatal infectious diseases known. The pathology is related to diarrheal diseases caused by the enteric bacteria, except it is relentless, and a patient can die rapidly from dehydration. The cholera toxin, which is the classic model of a bacterial enterotoxin, is also produced by some strains of E. coli.


Figure 6. Vibrio cholerae, the agent of Asiatic or epidemic cholera.



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The Gram-negative aerobic rods and cocci

This group consists of Gram-negative bacteria phenotypically related to members of the genus Pseudomonas.  Their metabolism is respiratory and never fermentative. Important human pathogens include Pseudomonas aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, Bordetella pertussis, Haemophilus influenzae, Legionella pneumophila, Brucella, Francisella, and a few others. Many bacteria in this physiological group are free-living in soil and water, and they play an important role in decomposition, biodegradation, and the C and N cycles. Also, many bacteria that are pathogens of plants are found in this group, including Pseudomonas, Xanthomonas and Agrobacterium.


Figure 7. Three looks at Pseudomonas, the head of the Gram-negative aerobic rods. A. Electron micrograph, negative stain. B. Scanning electron micrograph. C. Gram stain.

 Pseudomonas aeruginosa is the quintessential opportunistic pathogen of humans. It is a leading cause of hospital-acquired infections (nosocomial infections), and it is difficult to eradicate due to its resistance to most antimicrobial agents. There is probably no tissue that cannot become infected by Pseudomonas if the host defenses are weakened, and it is difficult to treat due to inherent and acquired resistance to antimicrobial agents. It is usually involved in soft tissue infections, urinary tract infections and pneumonia.

Whooping cough (or pertussis) is caused by  Bordetella pertussis. The disease is particularly serious in infants and young children and has a high mortality rate. Whooping cough is controlled by vaccination with the acellular pertussis vaccine, which is  usually given in association with diphtheria, tetanus and sometimes H. influenzae type b (Hib), as part of the childhood immunization program in the U.S.

Legionnaires' pneumonia is caused by Legionella pneumophila. This pneumonia, and the bacterium, were not discovered until 1976, when there was an outbreak of disease at a Legionnaire's meeting in Philadelphia. It took several months to find, culture and grow the bacterium. The incident was a wake-up call to public health officials that there were probably a lot of disease-producing bacteria in the environment that they know nothing about.

Neisseria gonorrhoeae causes the sexually-transmitted disease gonorrhea, and  Neisseria meningitidis is the agent of meningococcal meningitis. The neisseriae are discussed below with the pyogenic cocci.

Haemophilus influenzae is also a cause of meningitis, but the incidence of the disease has declined rapidly with the use of the Hib vaccine which began in 1994. Haemophilus is sometimes involved in infections of the upper respiratory tract, particularly the sinuses.

Brucellosis is a chronic debilitating infection in humans associated with reproductive failure in domestic animals. Person-to-person transmission of brucellae is extremely rare. Brucella abortus is the species usually involved in human disease. The primary reservoir of the organism is in cattle, although bison are sometimes wrongfully accused.

Enterics

Enteric bacteria are Gram-negative rods with facultative anaerobic metabolism that live in the intestinal tracts of animals in health and disease. This group consists of Escherichia coli and its relatives, the members of the family Enterobacteriaceae. Enteric bacteria are related phenotypically to several other genera of bacteria such as Pseudomonas and Vibrios.  Generally, a distinction can be made on the ability to ferment glucose; enteric bacteria all ferment glucose to acid end products while similar Gram-negative bacteria (e.g. pseudomonads) cannot ferment glucose. Because they are consistent members of the normal flora humans, and because of their medical importance, an extremely large number of enteric bacteria have been isolated and characterized.

Escherichia coli is, of course, the type species of the enterics. E. coli is such a regular inhabitant of the intestine of humans that it is used by public health authorities as an indicator of fecal pollution of drinking water supplies, swimming beaches, foods, etc. E. coli is the most studied of all organisms in biology because of its natural occurrence and the ease and speed of growing the bacterium in the laboratory. It has been used in hundreds of thousands of experiments in cell biology, physiology, and genetics, and was among the first cells for which the entire chromosomal DNA base sequence (genome) was determined. In spite of the knowledge gained about the molecular biology, genetics and physiology of E. coli, surprisingly little is known about its ecology, for example, why it consistently associates with humans, how it helps its host, how it harms its host, etc. A few strains of E. coli are pathogenic (one is now notorious, strain 0157:H7, that has been found to contaminate raw hamburger, vegetables, unpasteurized milk and drinking water) . Escherichia coli causes intestinal tract infections (usually acute and uncomplicated, except in the very young) or uncomplicated urinary tract infections and neonatal meningitis.


Figure 8. E. coli O157.H7. © David E. Graham. Virginia Polytechnic Institute and State University, Blacksburg, Virginia. Image by William Ghiorse, Department of Microbiology, Cornell University, Ithaca, New York. Licensed for use by ASM Microbe Library http://www.microbelibrary.org.This is a phase contrast image of cells immobilized on an agar-coated slide.

The enteric group includes two other important some other intestinal pathogens of humans: Salmonella and Shigella. Shigella dysenteriae causes bacillary dysentery: Salmonella enterica, causes food poisoning and gastroenteritis. Salmonella typhi, which infects via the intestinal route, causes typhoid fever.

Some bacteria that don't have an intestinal habitat resemble E. coli in enough ways to warrant inclusion in the enteric group. This includes Proteus, a common saprophyte of decaying organic matter and Yersinia pestis, which causes bubonic plague. Also classified as an enteric is  Erwinia, a pathogen of plants that causes fireblight in pear and apple trees and soft rot of carrots and potatoes.

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Pyogenic Cocci

The pyogenic cocci are spherical bacteria that cause various suppurative (pus-producing) infections in animals. Included are the Gram-positive cocci Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae, and the Gram-negative cocci, Neisseria gonorrhoeae and N. meningitidis. In terms of their phylogeny, physiology and genetics, these genera of bacteria are unrelated to one another. They share a common ecology, however, as parasites of humans.

The Gram-positive cocci are the leading pathogens of humans. It is estimated that they produce at least a third of all the bacterial infections of humans, including strep throat, pneumonia, otitis media, meningitis, food poisoning, various skin diseases and severe types of septic shock. The Gram-negative cocci, notably the neisseriae, cause gonorrhea and meningococcal meningitis.


Figure 9. Gallery of pyogenic cocci, Gram stains of clinical specimens (pus), L to R: Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria gonorrhoeae, Neisseria meningitidis. The large cells with lobed nuclei are neutrophils. Pus is the outcome of the battle between phagocytes (neutrophils) and the invading cocci. As the bacteria are ingested and killed by the neutrophils, the neutrophils eventually lyse (rupture) and release their own components, plus the digested products of bacterial cells, which are the make-up of pus. As a defense against phagocytes the staphylococci and streptococci produce toxins that kill the neutrophils before they are able to ingest the bacteria. This contributes to the pus, and therefore these bacteria are "pyogenic" during their pathogenic invasions.

Two species of Staphylococcus live in association with humans: Staphylococcus epidermidis which lives normally on the skin and mucous membranes, and Staphylococcus aureus, which may occur normally at various locales, but in particular on the nasal membranes (nares). S. epidermidis is rarely a pathogen and probably benefits its host by producing acids on the skin that retard the growth of dermatophytic fungi.

S. aureus always has the potential to cause disease and so is considered a pathogen. Different strains of S. aureus differ in the range of diseases they can cause, including boils and pimples, wound infections, pneumonia, osteomyelitis, septicemia, food intoxication, and toxic shock syndrome. S. aureus is the leading cause of nosocomial (hospital-acquired) infections by Gram-positive bacteria. Also, it is notoriously resistant to penicillin and many other antibiotics. Recently, a strain of S. aureus has been reported that is resistant to all known antibiotics in clinical usage, which is a grim reminder that the clock is ticking on the lifetime of the usefulness of current antibiotics in treatment of infectious disease.

Staphylococcus aureus is a successful bacterial pathogen because it has a very wide range of virulence determinants (structural, biochemical or genetic features that allow the bacterium to cause disease), and it occurs as normal flora of humans (on skin, nasal membranes and the GI tract), which ensures that it is readily transmitted from one individual to another.

Streptococcus pyogenes, more specifically the beta-hemolytic group A streptococci, like S. aureus, cause an array of suppurative diseases and toxinoses (diseases due to the production of a bacterial toxin), in addition to some autoimmune or allergic diseases. S. pyogenes is occasionally found as normal flora in the upper respiratory tract (<15% of individuals), but it is the main streptococcal pathogen for man, most often causing tonsillitis or strep throat. Streptococci also invade the skin to cause localized infections and lesions, and produce toxins that cause scarlet fever and toxic shock. Sometimes, as a result of an acute streptococcal infection, anomalous immune responses are started that lead to diseases like rheumatic fever and glomerulonephritis, which are called post-streptococcal sequelae. Unlike the staphylococci, the streptococci have not developed widespread resistance to penicillin and the other beta lactam antibiotics, so that the beta lactams remain drugs of choice for the treatment of acute streptococcal infections.

 Streptococcus pneumoniae is the most frequent cause of bacterial pneumonia in humans. It is also a frequent cause of otitis media (infection of the middle ear) and meningitis. The bacterium colonizes the nasopharynx and from there gains access to the lung or to the eustachian tube. If the bacteria descend into the lung they can impede engulfment by alveolar macrophages if they possess a capsule which somehow prevents the engulfment process. Thus, encapsulated strains are able to invade the lung and are virulent (cause disease), and noncapsulated strains, which are readily removed by phagocytes, are nonvirulent.

The Neisseriae cause gonorrhea and meningitis. Neisseriaceae is a family of Gram-negative bacteria with characteristics of enterics and pseudomonads. The neisseriae are small, Gram-negative cocci usually seen in pairs with flattened adjacent sides. Most neisseriae are normal flora or harmless commensals of mammals living on mucous membranes. In humans they are common residents of the throat and upper respiratory tract. Two species are primary pathogens of man, Neisseria gonorrhoeae and Neisseria meningitidis.

Neisseria gonorrhoeae is the second leading bacterial cause of sexually-transmitted disease in the U.S., causing over 300,000 cases of gonorrhea annually. Sometimes, in females, the disease may be unrecognized or asymptomatic such that an infected mother can give birth and unknowingly transmit the bacterium to the infant during its passage through the birth canal. The bacterium is able to colonize and infect the newborn eye resulting neonatal ophthalmia, which may produce blindness. For this reason (as well as to control Chlamydia which may also be present), an antimicrobial agent is usually added to the newborn eye at the time of birth.

Neisseria meningitidis is an important cause of bacterial meningitis, an inflammation of the meninges of the brain and spinal cord. Other bacteria that cause meningitis include Haemophilus influenzae, Staphylococcus aureus and Escherichia coli. Meningococcal meningitis differs from other causes in that it is often responsible for epidemics of meningitis. It occurs most often in children aged 6 to 11 months, but it also occurs in older children and in adults. Meningococcal meningitis can be a rapidly fatal disease, and untreated meningitis has a mortality rate near 50 percent. However, early intervention with antibiotics is highly effective, and with treatment most individuals recover without permanent damage to the nervous system.



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Endospore-forming bacteria

Endospore-forming bacteria produce a unique resting cell called an endospore. They are Gram-positive and usually rod-shaped, but there are exceptions. The two medically-important genera are Bacillus, the members of which are aerobic sporeformers in the soils, and Clostridium, whose species are anaerobic sporeformers of soils, sediments and the intestinal tracts of animals.


Figure 10. Endospore-forming bacilli (phase contrast illumination). Endospores are dehydrated, refractile cells appearing as points of bright light under phase microscopy. Endospore-forming bacteria are characterized by the location (position) of the endospore in the mother cell (sporangium) before its release. The spore may be central, terminal or subterminal, and the sporangium may or may not be swollen to accommodate the spore.

Some sporeformers are pathogens of animals, usually due to the production of powerful toxins. Bacillus anthracis causes anthrax, a disease of domestic animals (cattle, sheep, etc.),  which may be transmitted to humans.  Bacillus cereus causes food poisoning. Clostridium botulimum causes botulism, a form of food poisoning, and Clostridium tetani is the agent of tetanus. Clostridium perfringens causes food poisoning, anaerobic wound infections and gas gangrene, and Clostridium difficile causes a severe form of colitis called pseudomembranous colitis. Whenever the spore-formers act as pathogens, it is not uncommon or surprising that their spores are somehow involved in transmission or survival of the organism between hosts.


Figure 11. Robert Koch's original photomicrographs of Bacillus anthracis. In 1876, Koch established by careful microscopy that the bacterium was always present in the blood of animals that died of anthrax. He took a small amount of blood from such an animal and injected it into a healthy mouse, which subsequently became diseased and died. He took blood from that mouse and injected it into a another healthy mouse. After repeating this several times he was able to recover the original anthrax organism from the dead mouse, demonstrating for the first time that a specific bacterium is the cause of a specific disease. In so doing, he established Koch's Postulates, which still today supply the microbiological standard to demonstrate that a specific microbe is the cause of a specific disease.

Listeria monocytogenes is a Gram-positive rod-shaped bacterium related to Bacillus and Clostridium, but it does not form endospores. Listeria monocytogenes  is the agent of listeriosis, a serious infection caused by eating food contaminated with the bacteria.  Listeriosis has recently been recognized as an important public health problem in the United States. The disease affects primarily pregnant women, newborns, and adults with weakened immune systems.


Figure 12. Listeria monocytogenes. Transmission EM.

Actinomycetes and related bacteria

The actinomycetes are not thought of as pathogenic bacteria, but two of their relatives are among the most important pathogens of humans, these being the agents of tuberculosis and diphtheria. Actinomycetes are a large group of Gram-positive bacteria that usually grow by filament formation, or at least show a tendency towards branching and filament formation. Many of the organisms can form resting structures called spores, but they are not the same as endospores. Branched forms superficially resemble molds and are a striking example of convergent evolution of a procaryote and a eucaryote together in the soil habitat. Actinomycetes such as Streptomyces have a world-wide distribution in soils. They are important in aerobic decomposition of organic compounds and have an important role in biodegradation and the carbon cycle. Actinomycetes are the main producers of antibiotics in industrial settings, being the source of most tetracyclines, macrolides (e.g. erythromycin), and aminoglycosides (e.g. streptomycin, gentamicin, etc.).

Two genera of  bacteria that are related to the actinomycetes, Corynebacterium and Mycobacterium, contain important pathogens of humans: Otherwise, many nonpathogenic mycobacteria and corynebacteria live in normal associations with animals.

Mycobacterium tuberculosis is the etiologic agent of tuberculosis (TB) in humans. Tuberculosis is the leading cause of death in the world from a single infectious disease. Mycobacterium tuberculosis infects 1.7 billion people/year which is equal to 33% of the entire world population. The bacterium is responsible for over 3 million deaths/year. After a century of decline in the United States, cases of tuberculosis have increased slightly, and multiple drug-resistant strains have emerged,  This increase in cases is attributable to changes in the social structure in cities, the HIV epidemic, and patient failure to comply with treatment programs. A related organism, Mycobacterium leprae, causes leprosy.


Figure 13. Mycobacterium tuberculosis Acid-fast stain. 1000X magnification. ©  Gloria J. Delisle and Lewis Tomalty, Queens University, Kingston, Ontario, Canada. Licensed for use by ASM Microbe Library http://www.microbelibrary.org.These bacteria were observed in a sputum sample from a patient with active tuberculosis.

The genus Corynebacterium consists of a diverse group of bacteria including animal and plant pathogens, as well as saprophytes. Some corynebacteria are part of the normal flora of humans, finding a suitable niche in virtually every anatomic site. The best known and most widely studied species is  Corynebacterium diphtheriae, the causal agent of diphtheria.  The study of Corynebacterium diphtheriae traces closely the development of medical microbiology, immunology and molecular biology. Many contributions to these fields, as well as to our understanding of host-bacterial interactions, have been made studying diphtheria and the diphtheria toxin.

Rickettsias and chlamydiae are two unrelated groups of bacteria that are obligate intracellular parasites of eucaryotic cells. Rickettsias cannot grow outside of a host cell because they have leaky membranes and are unable to obtain nutrients in an extracellular habitat. Chlamydiae are unable to produce ATP in amounts required to sustain metabolism outside of a host cell and are, in a sense, energy-parasites.

Rickettsias occur in nature in the gut lining of arthropods (ticks, fleas, lice, etc.). They are transmitted to vertebrates by an arthropod bite and produce diseases such as typhus fever, Rocky Mountain Spotted Fever, Q fever and ehrlichiosis.  See Rickettsial Diseases, including Rocky Mountain Spotted Fever.

Chlamydiae are tiny bacteria that infect birds and mammals. They may colonize and infect tissues of the eye and urogenital tract in humans. Chlamydia trachomatis causes several important diseases in humans: chlamydia, the most prevalent sexually transmitted disease in the U.S., trachoma, a leading cause of blindness worldwide, and lymphogranuloma venereum. Chlamydia pneumoniae is a cause of pneumonia and has been recently linked to atherosclerosis.


Figure 14. Ehrlichia chaffeensis © Vsevolod Popov, J. Steven Dumler, and David H. Walker. University of Texas Medical Branch at Galveston. Licensed for use by ASM Microbe Library http://www.microbelibrary.org. Ehrlichia are obligate intracellular parasites  related to the rickettsiae that are tick-borne pathogens of dogs and humans. In humans, they cause human granulocytic ehrlichiosis (HGE) and human monocytic ehrlichiosis (HME). In this electron micrograph, dense-core cells of E. chaffeensis are seen exiting the host cell following rupture of the cytoplasmic membrane. The ehrlichiae will now go on to infect additional host cells or they may be ingested by a feeding tick, and spread to another animal.

Mycoplasmas are a group of bacteria that lack a cell wall. The cells are bounded by a single triple-layered membrane. They may be free-living in soil and sewage, parasitic inhabitants of the mouth and urinary tract of humans, or pathogens in animals and plants. In humans, Mycoplasma pneumoniae causes primary atypical pneumonia, also called walking pneumonia.



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Table 2. Important Bacteria that Are Pathogens of Humans
Bacterial pathogen
  
Disease(s)
Transmission
Gram-negative bacteria 


Escherichia coli
Gastroenteritis, urinary tract infections, neonatal meningitis
F W E
E. coli O157:H7
Diarrhea, hemolytic uremic syndrome (HUS)
F
Salmonella enterica
Gastroenteritis
F W
Salmonella typhi

Typhoid fever
F W
Shigella dysenteriae
Bacillary dysentery
F W
Yersina pestis
Bubonic plague
IV
Pseudomonas aeruginosa
Opportunistic infections, swimmer´┐Żs ear, hot tub itch, cellulitis, pneumonia, more
S W C HA E
Vibrio cholerae Asiatic cholera
W
Bordetella pertussis
Whooping cough
RC
Haemophilus influenzae
Meningitis, pneumonia, sinusitis
RC
Helicobacter pylori
Gastric and duodenal ulcers
F?
Campylobacter jejuni
Gastroenteritis
F W
Neisseria gonorrhoeae
Gonorrhea
S C
Neisseria meningitidis
Meningococcemia and meningitis
R C E
Brucella abortus
Undulant fever
IA M
Bacteroides fragilis
Anaerobic infections
E

Gram-positive bacteria


Staphylococcus aureus
Food poisoning, wound infections, toxic shock syndrome, more F C E HA IA
Streptococcus pyogenes
Strep throat, scarlet fever, mastitis, necrotizing fasciitis, more   
C
Streptococcus pneumoniae
Pneumonia, otitis media, meningitis
RC E
Bacillus anthracis
Anthrax
S IA
Bacillus cereus
Food poisoning
F
Clostridium tetani
Tetanus
S
Clostridium perfringens
Food poisoning, gas gangrene, uterine infections
F S E
Clostridium botulinum
Botulism, infant botulism
F
Clostridium difficile
Antibiotic-associated diarrhea, pseudomembranous colitis
C HA E
Corynebacterium diphtheriae Diphtheria
RC
Listeria monocytogenes Listeriosis
F
Not typed by Gram stain


Mycobacterium tuberculosis       
TB (tuberculosis)
RC M
Mycobacterium leprae
Leprosy
C
Chlamydia trachomatis Chlamydia, lymphogranuloma venereum, trachoma
SC C
Chlamydia pneumoniae Pneumonia
RC
Mycoplasma pneumoniae
Atypical pneumonia
RC
Rickettisas Rickettsiosis: typhus, RMSF 
IV
Treponema pallidum Syphilis
SC
Borrelia burgdorferi Lyme disease IV

KEY TO TRANSMISSION. C = Contact   E = Endogenous   F = Food borne   HA = Hospital Acquired
IA = Infected Animal   IV = Insect Vector   M = Milk   RC = Respiratory Contact   SC = Sexual Contact
S = Soil   W = Water


Table 3. Bacterial Diseases of Humans by Anatomical Site or Type

Infections of the oral cavity
Dental caries: Streptococcus mutans, S. oralis, S. sanguis, S. gordonii
Gingivitis
Periodontal disease

GI Tract
Gastric and duodenal ulcers: Helicobacter pylori
Gastroenteritis: Salmonella, Campylobacter, E. coli
Dysentery: Shigella
Antibiotic´┐Żassociated diarrhea and pseudomembranous colitis: Clostridium difficile
Asiatic cholera: Vibrio cholerae
Food poisoning
    Staphylococcus aureus
    Bacillus cereus
    Clostridium perfringens
    Clostridium botulinum
Food infections
    Salmonella
    E. coli O157:H7
    Campylobacter jejuni
    Listeria monocytogenes: Listeriosis

Respiratory Tract
Sinusitis
    Haemophilus influenzae
    Pseudomonas aeruginosa
Strep throat: Streptococcus pyogenes
Diphtheria: Corynebacterium diphtheriae
Pneumonia
    Streptococcus pneumoniae
    Staphylococcus aureus
    Pseudomonas aeruginosa
    Haemophilus influenzae
    Chlamydia pneumoniae
    Mycoplasma pneumoniae
Whooping cough: Bordetella pertussis
Tuberculosis: Mycobacterium tuberculosis

Sexually-Transmitted Disease
Chlamydia: Chlamydia trachomatis
Gonorrhea: Neisseria gonorrhoeae
Syphilis: Treponema pallidum

Urinary Tract Infections
    E. coli
    Pseudomonas aeruginosa
    Staphylococcus aureus

Meningitis
    Neisseria meningitidis
    Haemophilus influenzae
    Streptococcus pneumoniae
    Escherichia coli

Otitis
    Otitis externa: Pseudomonas aeruginosa
    Otitis media: Streptococcus pneumoniae, Haemophilus influenzae

Skin
Acne, boils, pimples, impetigo: Staphylococcus aureus
Hot tub itch, folliculitis, cellulitis: Pseudomonas aeruginosa
Necrotizing fasciitis: Streptococcus pyogenes

Zoonoses
Anthrax: Bacillus anthracis
Brucellosis: Brucella abortus
Leptospirosis: Leptospira

Arthropod borne
Lyme disease: Borrelia burgdorferi
Ehrlichiosis: Ehrlichia
Rickettsiosis (Typhus, Rocky Mountain Spotted Fever): Rickettsias
Plague: Yersinia pestis

Toxinoses
Toxic shock syndrome, Scalded skin syndrome: Staphylococcus aureus
Scarlet fever: Streptococcus pyogenes
Hemolytic Uremic Syndrome (HUS): E. coli O157:H7
Diphtheria: Corynebacterium diphtheriae
Anthrax: Bacillus anthracis
Tetanus: Clostridium tetani
Botulism: Clostridium botulinum

Hospital-acquired (nosocomial) infections
MRSA (methicillin-resistant Staphylococcus aureus)
Escherichia coli
Pseudomonas aeruginosa
Clostridium difficile




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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

© 2008 Kenneth Todar, PhD - Madison, Wisconsin