Medically Important Bacteria

Bacteroides species

Obligate anaerobes that commonly inhabit the mouth, intestinal tract, and genital tract. Cause Abscesses and bloodstream infections

Bordetella pertussis

Causes pertussis (whooping cough)


Most live in the intestinal tract

Enterobacter species

Normal microbiota of the intestinal tract

Escheriachia coli

Normal microbiota of the intestinal tract. Some strains cause urinary tract infections; some strains cause specific types of intestinal disease; some cause meningitis in newborns

Klebsiella pneumoniae

Normal microbiota of the intestinal tract. Causes pneumonia

Proteus species

Normal microbiota of the intestinal tract. Cause urinary tract infections

Salmonella enterica
serotype Enteritidis

Causes gastroenteritis. Grows in the intestinal tract of infected animals; acquired by consuming contaminated food

Salmonella enterica
serotype Typhi

Causes typhoid fever. Grows in the intestinal tract of infected humans; transmitted in feces

Shigella species

Cause dysentery. Grow in the intestinal tract of infected humans; transmitted in feces

Yersinia pestis

Causes bubonic plague, which is transmitted by fleas, and pneumonic plague, which is transmitted in respiratory droplets of infected individuals

Haemophilus influenzae

Causes ear infections, respiratory infections, and meningitis in children

Haemophilus ducreyi

Causes chancroid, a sexually transmitted disease

Legionella pneumophila

Causes Legionnaires' disease, a lung infection. Grows within protozoa; acquired by inhaling contaminated water droplets

Pseudomonas aeruginosa

Causes burn, urinary tract, and bloodstream infections. Common in the environment. Grows in nutrient-poor aqueous solutions. Resistant to many disinfectants and antimicrobial medications

Chlamydophila (Chlamydia) pneumoniae

Causes atypical pneumonia, or "walking pneumonia." Acquired from an infected person

Chlamydophila (Chlamydia) psittaci

Causes psittacosis, a form of pneumonia. Transmitted by birds

Chlamydia trachomatis

Causes a sexually transmitted disease that mimics the symptoms of gonorrhea. Also causes trachoma, a serious eye infection, and conjunctivitis in newborns

Coxiella burnetii

Causes Q fever. Acquired by inhaling organisms shed by infected animals

Ehrlichia chaffeensis

Causes human ehrlichiosis. Transmitted by ticks

Orientia tsutsugamushi

Causes scrub typhus. Transmitted by mites

Rickettsia prowazekii

Causes epidemic typhus. Transmitted by lice

Rickettsia rickettsii

Causes Rocky Mountain spotted fever. Transmitted by ticks

Wolbachia pipientis

Resides within the filarial worms that cause river blindness and elephantiasis

Campylobacter jejuni

Causes gastroenteritis. Grows in the intestinal tract of infected animals; acquired by consuming contaminated food

Helicobacter pylori

Causes stomach and duodenal ulcers. Neutralizes stomach acid by producing urease

Vibrio cholerae

Causes cholera, a severe diarrheal disease. Grows in the intestinal tract of infected humans; acquired by drinking contaminated water

Vibrio parahaemolyticus

Causes gastroenteritis. Acquired by consuming contaminated seafood

Vibrio vulnificus

Causes a systemic disease, particularly in people who have liver failure or other underlying complications

Neisseria meningitidis

Causes meningitis

Neisseria gonorrhoeae

Causes gonorrhea, a sexually transmitted infection

Bacillus anthracis

Causes anthrax. Acquired by inhaling endospores in soil, animal hides, and wool. Also acquired by touching or ingesting the endospores. Bioterrorism agent

Bifidobacterium species

Predominant member of the intestinal tract in breast-fed infants. Thought to play a protective role in the intestinal tract by excluding pathogens

Clostridium botulinum

Causes botulism. Disease results from ingesting toxin-contaminated foods, typically canned foods that have been improperly processed

Clostridium perfringens

Causes gas gangrene. Acquired when soil-borne endospores contaminate a wound

Clostridium tetani

Causes tetanus. Acquired when soil-borne endospores are inoculated into deep tissue

Clostridium difficile

Causes Clostridium difficile infection (CDI), which is associated with antibiotic use and can result in severe diarrhea

Corynebacterium diphtheriae

Toxin-producing strains cause diphtheria

Enterococcus species

Normal microbiota of the intestinal tract. Cause urinary tract infections

Micrococcus species

Found on skin as well as in a variety of other environments; often contaminate bacteriological media

Staphylococcus aureus

Leading cause of wound infections. Causes boils, carbuncles, food poisoning, and toxic shock syndrome

Staphylococcus epidermidis

Normal microbiota of the skin

Staphylococcus saprophyticus

Causes urinary tract infections

Streptococcus pneumoniae

Causes pneumonia and meningitis

Streptococcus pyogenes

Causes pharyngitis (strep throat), rheumatic fever, wound infections, glomerulonephritis, and streptococcal toxic shock

Mycobacterium tuberculosis

Causes tuberculosis

Mycobacterium leprae

Causes Hansen's disease (leprosy); peripheral nerve invasion is characteristic

Treponema pallidum

Causes syphilis, a sexually transmitted disease. the organism has never been grown in culture

Borrelia burgdorferi

Causes Lyme disease, a tick-borne disease

Borrelia recurrent is and B. hermsii

Causes relapsing fever. Transmitted by arthropods

Leptospira interrogans

Cause leptospirosis, a waterborne disease. Excreted in urine of infected animals

Mycoplasma pneumoniae

Causes atypical pneumonia ("walking pneumonia"). Not susceptible to penicillin because it lacks a cell wall