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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 (page 3)

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© Kenneth Todar, PhD

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.


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 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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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.

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