Bacterial Pathogens of Humans
(This chapter has 6 pages)
© 2008 Kenneth Todar, PhD
Historically, bacteria have been the cause of some of the most
diseases and widespread epidemics of human civilization. Smallpox
and malaria, diseases caused by other microbes, have killed more
than bacterial diseases, but diseases such as tuberculosis,
plague, diphtheria, typhoid, cholera, dysentery and pneumonia
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
and antibiotic treatment have reduced the morbidity and
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 2. CDC.
some bacterial diseases have been conquered (for the present), but
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
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
and diseases recognized or reemerged since 1977
||gastroenteritis distributed world-wide
||toxic shock syndrome
|E. coli O157:H7
||hemorrhagic colitis; hemolytic uremic syndrome
||Lyme Disease and complications
||gastric and duodenal ulcers
||antibiotic induced diarrhea; pseudomembranous
|Vibrio cholerae O139
|Salmonella enterica Serotype
Typhimurium DT 104
||cat scratch fever
|Streptococcus pyogenes (Group A Strep)
|necrotizing fasciitis (GAS); streptococcal
resistant S. aureus (e.g.
||sudden infant death syndrome (SIDS)
||wound infection, septicemia, gastrointestinal
|blood stream infections
enterococci (E. faecalis and E. faecium)
resistant Acinetobacter baumannii
PAGE 2 STARTS HERE
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
criteria, rather than on the basis of affected organ, mode
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
chlamydiae, spirochetes and the mycobacteria.
The spirochetes are a phylogenetically distinct group of bacteria
have a unique cell morphology and mode of motility. Spirochetes are
thin, flexible, spiral-shaped bacteria that move by means of
called axial filaments or endoflagella. The flagellar filaments are
within a sheath between the cell wall peptidoglycan and an outer
The filaments flex or rotate within their sheath, which causes the
to bend, flex and rotate during movement. Most spirochetes are free
(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.
section of a spirochete showing the location of endoflagella between
inner membrane and outer sheath; B.
Borrelia burgdorferi, the agent
of Lyme disease; C. Treponema pallidum, the spirochete that
Spirilla and other curved bacteria
Spirilla are Gram-negative bacteria with a helical or spiral shape.
Their metabolism is respiratory and never fermentative. Unlike
they have a rigid cell wall and are motile by means of ordinary polar
Two important pathogens of humans are found among the spiral forms.
jejuni is the cause of bacterial diarrhea, especially in
children. The bacterium is transmitted via contaminated food, usually
poultry or shellfish, or untreated drinking water.
pylori is able to colonize the gastric mucosal cells of
i.e., the lining of the stomach, and it has been well established as
cause of peptic
ulcers and there is strong evidence for its involvement in
Figure 5. Helicobacter
The term vibrio refers to a Gram-negative bacterium which
the cell shape of a curved rod or a comma. Members of the genus Vibrio
consists of common bacteria in aquatic environments, especially marine
They have structural and metabolic properties that overlap with both
enterics and the pseudomonads. Vibrios are facultative anaerobes (grow
or absence of O2), like enterics, but they have polar
are oxidase-positive, and degrade sugars in the same manner as the
In aquatic habitats, they overlap with the pseudomonads in their
although pseudomonads favor fresh water and vibrios prefer salt water.
Some marine vibrios are bioluminescent (they emit light) and some are
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
except it is relentless, and a patient can die rapidly from
The cholera toxin, which is the classic model of a bacterial
is also produced by some strains of E. coli.
Figure 6. Vibrio cholerae,
the agent of Asiatic or epidemic cholera.
PAGE 3 STARTS HERE
The Gram-negative aerobic rods and cocci
This group consists of Gram-negative bacteria phenotypically related
members of the genus Pseudomonas. Their metabolism is
and never fermentative. Important human pathogens include Pseudomonas
aeruginosa, Neisseria gonorrhoeae, Neisseria meningitidis, Bordetella
Haemophilus influenzae, Legionella pneumophila, Brucella, Francisella,
and a few others. Many bacteria in this physiological group are
in soil and water, and they play an important role in decomposition,
and the C and N cycles. Also, many bacteria that are pathogens of
are found in this group, including Pseudomonas, Xanthomonas
Figure 7. Three looks at Pseudomonas,
head of the Gram-negative aerobic rods. A. Electron micrograph,
stain. B. Scanning electron micrograph. C. Gram stain.
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
infected by Pseudomonas if the host defenses are weakened, and
is difficult to treat due to inherent and acquired resistance to
It is usually involved in soft tissue infections, urinary tract
Whooping cough (or pertussis) is caused by Bordetella
pertussis. The disease is particularly serious in infants
young children and has a high mortality rate. Whooping cough is
by vaccination with the acellular pertussis vaccine, which
usually given in association with diphtheria, tetanus and sometimes H.
influenzae type b (Hib), as part of the childhood immunization
in the U.S.
Legionnaires' pneumonia is caused by Legionella
This pneumonia, and the bacterium, were not discovered until 1976, when
there was an outbreak of disease at a Legionnaire's meeting in
It took several months to find, culture and grow the bacterium. The
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
gonorrhoeae causes the sexually-transmitted disease gonorrhea,
meningitidis is the agent of meningococcal meningitis.
The neisseriae are discussed below with the pyogenic cocci.
influenzae is also a cause of meningitis, but the
of the disease has declined rapidly with the use of the Hib vaccine
began in 1994. Haemophilus is sometimes involved in infections
the upper respiratory tract, particularly the sinuses.
Brucellosis is a chronic debilitating infection in humans
with reproductive failure in domestic animals. Person-to-person
of brucellae is extremely rare. Brucella abortus is the
usually involved in human disease. The primary reservoir of the
is in cattle, although bison are sometimes wrongfully accused.
bacteria are Gram-negative rods with facultative anaerobic
that live in the intestinal tracts of animals in health and disease.
group consists of Escherichia coli and its relatives,
members of the family Enterobacteriaceae. Enteric
are related phenotypically to several other genera of bacteria such as
and Vibrios. Generally, a distinction can be made on the ability
to ferment glucose; enteric bacteria all ferment glucose to acid end
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
large number of enteric bacteria have been isolated and characterized.
Escherichia coli is, of course, the type species of
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
of its natural occurrence and the ease and speed of growing the
the laboratory. It has been used in hundreds of thousands of
in cell biology, physiology, and genetics, and was among the first
for which the entire chromosomal DNA base sequence (genome) was
In spite of the knowledge gained about the molecular biology, genetics
of E. coli, surprisingly little is known about its ecology, for
example, why it consistently associates with humans, how it helps its
how it harms its host, etc. A few strains of E. coli are
(one is now notorious, strain 0157:H7, that has been found to
hamburger, vegetables, unpasteurized milk and drinking water) .
coli causes intestinal tract infections
acute and uncomplicated, except in the very young) or uncomplicated
urinary tract infections and neonatal meningitis.
Figure 8. E. coli
© David E. Graham. Virginia Polytechnic Institute and State
Blacksburg, Virginia. Image by William Ghiorse, Department of
Cornell University, Ithaca, New York. Licensed for use by ASM Microbe
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
Shigella dysenteriae causes bacillary
dysentery: Salmonella enterica, causes food
poisoning and gastroenteritis.
typhi, which infects via the intestinal route, causes typhoid
Some bacteria that don't have an intestinal habitat resemble
coli in enough ways to warrant inclusion in the enteric group. This
includes Proteus, a common saprophyte of decaying
matter and Yersinia pestis, which causes
Also classified as an enteric is Erwinia, a
of plants that causes fireblight in pear and apple trees and soft rot
carrots and potatoes.
PAGE 4 STARTS HERE
The pyogenic cocci are spherical bacteria that cause various
suppurative (pus-producing) infections in animals. Included are the
Staphylococcus aureus, Streptococcus pyogenes and Streptococcus
pneumoniae, and the Gram-negative cocci, Neisseria
and N. meningitidis. In terms of their phylogeny,
and genetics, these genera of bacteria are unrelated to one another.
share a common ecology, however, as parasites of humans.
The Gram-positive cocci are the leading pathogens of humans. It is
that they produce at least a third of all the bacterial infections of
including strep throat, pneumonia, otitis media, meningitis, food
poisoning, various skin
and severe types of septic shock. The Gram-negative cocci, notably
the neisseriae, cause gonorrhea and
Figure 9. Gallery of pyogenic
cocci, Gram stains of clinical specimens (pus), L to R: Staphylococcus
aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Neisseria
Neisseria meningitidis. The large cells with lobed nuclei are
Pus is the outcome of the battle between phagocytes (neutrophils) and
invading cocci. As the bacteria are ingested and killed by the
the neutrophils eventually lyse (rupture) and release their own
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
on the nasal membranes (nares). S. epidermidis is rarely
a pathogen and probably benefits its host by producing acids on the
that retard the growth of dermatophytic fungi.
S. aureus always has the potential to cause disease
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
and toxic shock syndrome. S. aureus is the leading
of nosocomial (hospital-acquired) infections by Gram-positive
Also, it is notoriously resistant to penicillin and many other
Recently, a strain of S. aureus has been reported that is
to all known antibiotics in clinical usage, which is a grim reminder
the clock is ticking on the lifetime of the usefulness of current
in treatment of infectious disease.
Staphylococcus aureus is a successful bacterial pathogen
it has a very wide range of virulence determinants (structural,
biochemical or genetic features that allow the bacterium to cause
and it occurs as normal
flora of humans (on skin, nasal membranes and the GI tract),
ensures that it is readily transmitted from one individual to another.
pyogenes, more specifically the beta-hemolytic group A
like S. aureus, cause an array of suppurative diseases
(diseases due to the production of a bacterial toxin), in addition to
diseases. S. pyogenes is occasionally found as normal
flora in the upper respiratory tract (<15% of individuals),
it is the main streptococcal pathogen for man, most often causing tonsillitis
strep throat. Streptococci also invade the skin to cause
infections and lesions, and produce toxins that cause scarlet fever
and toxic shock. Sometimes, as a result of an acute streptococcal
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
widespread resistance to penicillin and the other beta lactam
so that the beta lactams remain drugs of choice for the treatment of
pneumoniae is the most frequent cause of
in humans. It is also a frequent cause of otitis media
of the middle ear) and meningitis. The bacterium colonizes the
and from there gains access to the lung or to the eustachian tube. If
bacteria descend into the lung they can impede engulfment by alveolar
if they possess a capsule which somehow prevents the engulfment
Thus, encapsulated strains are able to invade the lung and are virulent
(cause disease), and noncapsulated strains, which are readily removed
phagocytes, are nonvirulent.
The Neisseriae cause gonorrhea
and meningitis. Neisseriaceae is a family of
bacteria with characteristics of enterics and pseudomonads. The
are small, Gram-negative cocci usually seen in pairs with flattened
sides. Most neisseriae are normal
flora or harmless commensals of mammals living on mucous
In humans they are common residents of the throat and upper respiratory
tract. Two species are primary pathogens of man,
Neisseria gonorrhoeae and
gonorrhoeae is the second leading bacterial cause of
disease in the U.S., causing over 300,000 cases of gonorrhea
Sometimes, in females, the disease may be unrecognized or asymptomatic
such that an infected mother can give birth and unknowingly transmit
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
to control Chlamydia which may also be present), an antimicrobial agent
is usually added to the newborn eye at the time of birth.
meningitidis is an important cause of bacterial meningitis,
an inflammation of the meninges of the brain and spinal cord. Other
that cause meningitis include Haemophilus influenzae,
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,
it also occurs in older children and in adults. Meningococcal
can be a rapidly fatal disease, and untreated meningitis has a
rate near 50 percent. However, early intervention with antibiotics is
effective, and with treatment most individuals recover without
damage to the nervous system.
PAGE 5 STARTS HERE
Endospore-forming bacteria produce a unique resting cell called an endospore.
They are Gram-positive and usually rod-shaped, but there are
The two medically-important genera are Bacillus,
the members of which are aerobic sporeformers in the soils, and Clostridium,
species are anaerobic sporeformers of soils, sediments and the
tracts of animals.
Figure 10. Endospore-forming
bacilli (phase contrast illumination). Endospores are dehydrated,
cells appearing as points of bright light under phase microscopy.
bacteria are characterized by the location (position) of the endospore
in the mother cell (sporangium) before its release. The spore may be
terminal or subterminal, and the sporangium may or may not be swollen
accommodate the spore.
Some sporeformers are pathogens of animals, usually due to the
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
tetani is the agent of tetanus. Clostridium
causes food poisoning, anaerobic wound infections and gas
gangrene, and Clostridium difficile causes a severe
of colitis called pseudomembranous colitis. Whenever the
act as pathogens, it is not uncommon or surprising that their spores
somehow involved in transmission or survival of the organism between
Figure 11. Robert Koch's
photomicrographs of Bacillus anthracis. In 1876, Koch
by careful microscopy that the bacterium was always present in the
of animals that died of anthrax. He took a small amount of blood from
an animal and injected it into a healthy mouse, which subsequently
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
recover the original anthrax organism from the dead mouse,
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
supply the microbiological standard to demonstrate that a specific
is the cause of a specific disease.
Listeria monocytogenes is a Gram-positive rod-shaped
related to Bacillus and Clostridium, but it does not form
monocytogenes is the agent of listeriosis, a
infection caused by eating food contaminated with the bacteria.
has recently been recognized as an important public health problem in
United States. The disease affects primarily pregnant women, newborns,
and adults with weakened immune systems.
Figure 12. Listeria
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
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
and filament formation. Many of the organisms can form resting
called spores, but they are not the same as endospores. Branched forms
superficially resemble molds and are a striking example of convergent
of a procaryote and a eucaryote together in the soil habitat.
such as Streptomyces have a world-wide distribution in
They are important in aerobic decomposition of organic compounds and
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
(e.g. streptomycin, gentamicin, etc.).
Two genera of bacteria that are related to the actinomycetes,
and Mycobacterium, contain important pathogens of
many nonpathogenic mycobacteria and corynebacteria live in normal
tuberculosis is the etiologic agent of tuberculosis
(TB) in humans. Tuberculosis is the leading cause of death in
world from a single infectious disease. Mycobacterium
infects 1.7 billion people/year which is equal to 33% of the entire
population. The bacterium is responsible for over 3 million
After a century of decline in the United States, cases of tuberculosis
have increased slightly, and multiple drug-resistant strains have
in cases is attributable to changes in the social structure in cities,
the HIV epidemic, and patient failure to comply with treatment
A related organism, Mycobacterium leprae, causes leprosy.
Figure 13. Mycobacterium
tuberculosis Acid-fast stain. 1000X magnification. ©
J. Delisle and Lewis Tomalty, Queens University, Kingston, Ontario,
Licensed for use by ASM Microbe Library
bacteria were observed in a sputum sample from a patient with active
The genus Corynebacterium consists of a diverse group
of bacteria including animal and plant pathogens, as well as
Some corynebacteria are part of the normal flora of humans, finding a
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
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
Rickettsias and chlamydiae are
unrelated groups of bacteria that are obligate intracellular
eucaryotic cells. Rickettsias cannot grow outside of a host
because they have leaky membranes and are unable to obtain nutrients in
an extracellular habitat. Chlamydiae are unable to produce ATP
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
fleas, lice, etc.). They are transmitted to vertebrates by an arthropod
bite and produce diseases such as typhus
Spotted Fever, Q fever and ehrlichiosis. See Rickettsial
Diseases, including Rocky Mountain Spotted Fever.
Chlamydiae are tiny bacteria that infect birds and mammals.
may colonize and infect tissues of the eye and urogenital tract in
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
© Vsevolod Popov, J. Steven Dumler, and David H. Walker.
of Texas Medical Branch at Galveston. Licensed for use by ASM Microbe
Ehrlichia are obligate intracellular parasites related to the
rickettsiae that are tick-borne pathogens of dogs and humans. In
they cause human granulocytic ehrlichiosis (HGE) and human monocytic
(HME). In this electron micrograph, dense-core cells of E.
seen exiting the host cell following rupture of the cytoplasmic
The ehrlichiae will now go on to infect additional host cells or they
be ingested by a feeding tick, and spread to another animal.
Mycoplasmas are a group of
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
of the mouth and urinary tract of humans, or pathogens in animals and
In humans, Mycoplasma pneumoniae causes primary
pneumonia, also called walking pneumonia.
PAGE 6 STARTS HERE
Table 2. Important Bacteria that Are Pathogens of Humans
infections, neonatal meningitis
|F W E
| Typhoid fever
| F W
swimmer�s ear, hot tub itch, cellulitis, pneumonia, more
|S W C HA E
| F W
| S C
|R C E
infections, toxic shock syndrome, more
||F C E HA IA
mastitis, necrotizing fasciitis, more
| Food poisoning,
| F S E
|C HA E
typed by Gram stain
| SC C
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
S = Soil W = Water
Bacterial Diseases of Humans by Anatomical Site or Type
Infections of the oral cavity
Dental caries: Streptococcus mutans,
S. oralis, S. sanguis, S. gordonii
Gastric and duodenal ulcers: Helicobacter
Campylobacter, E. coli
Antibiotic�associated diarrhea and pseudomembranous colitis:
Asiatic cholera: Vibrio cholerae
E. coli O157:H7
Strep throat: Streptococcus pyogenes
Whooping cough: Bordetella pertussis
Chlamydia: Chlamydia trachomatis
Gonorrhea: Neisseria gonorrhoeae
Syphilis: Treponema pallidum
Urinary Tract Infections
Otitis externa: Pseudomonas
Otitis media: Streptococcus
Acne, boils, pimples, impetigo: Staphylococcus
Hot tub itch, folliculitis, cellulitis: Pseudomonas aeruginosa
Necrotizing fasciitis: Streptococcus
Anthrax: Bacillus anthracis
Brucellosis: Brucella abortus
Lyme disease: Borrelia burgdorferi
Rickettsiosis (Typhus, Rocky Mountain Spotted Fever): Rickettsias
Plague: Yersinia pestis
Toxic shock syndrome, Scalded skin syndrome: Staphylococcus aureus
Scarlet fever: Streptococcus pyogenes
Hemolytic Uremic Syndrome (HUS): E.
Anthrax: Bacillus anthracis
Tetanus: Clostridium tetani
Botulism: Clostridium botulinum
MRSA (methicillin-resistant Staphylococcus