Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic. One of the bacterial diseases with highest disease burden is tuberculosis, caused by the bacterium Mycobacterium tuberculosis, which kills about 2 million people a year, mostly in sub-Saharan Africa. Pathogenic bacteria contribute to other globally important diseases, such as pneumonia, which can be caused by bacteria such as Streptococcus and Pseudomonas, and foodborne illnesses, which can be caused by bacteria such as Shigella, Campylobacter and Salmonella. Pathogenic bacteria also cause infections such as tetanus, typhoid fever, diphtheria, syphilis and leprosy.
Each pathogenic species has a characteristic spectrum of interactions with its human hosts.
Clinically most important human pathogenic bacteria genera[8] and species | Genus | Important species | Gram staining | Shape | Capsulation | Bonding tendency | Motility | Respiration | Growth medium | Intra/Extracellular |
| Bordetella | | Gram-negative | Small coccobacilli | Encapsulated | singly or in pairs | | aerobic | Regan-Lowe agar | extracellular |
| Borrelia | | Gram-negative, but stains poorly | | | Long, slender, flexible, spiral- or corkscrew-shaped rods | highly motile | (difficult to culture) | | extracellular |
| Brucella | | Gram-negative | Small coccobacilli | Unencapsulated | singly or in pairs | | aerobic | Blood agar | intracellular |
| Campylobacter | | Gram-negative | Curved, spiral, or S-shaped with single, polar flagellum | | | characteristic darting motion | microaerophilic | Blood agar inhibiting other fecal flora | extracellular |
| Chlamydia | | (not Gram-stained) | Small, round, ovoid | | | | motile | Facultative or strictly aerobic | Obligate intracellular |
| Clostridium | | Gram-positive | Large, blunt-ended rods | | | mostly motile | Obligate anaerobic | Anaerobic blood agar | extracellular |
| Corynebacterium | | Gram-positive (unevenly) | Small, slender, pleomorphic rods | unencapsulated | clumps looking like Chinese characters or a picket fence | nonmotile | Mostly facultative anaerobic | Aerobically on Tinsdale agar | extracellular |
| Enterococcus | | Gram-positive | Round to ovoid | | pairs or chains | | 6.5% NaCl, bile-esculin agar | | extracellular |
| Escherichia | | Gram-negative | Short rods | | | | Facultative anaerobic | MacConkey agar | extracellular |
| Francisella | | Gram-negative | Small, pleomorphic coccobacillus | | | | strictly aerobic | (rarely cultured) | Facultative intracellular |
| Haemophilus | | Gram-negative | Ranging from small coccobacillus to long, slender filaments | | | | | Chocolate agar with hemin and NAD+ | extracellular |
| Helicobacter | | Gram-negative | Curved or spiral rods pultiple polar flagella | | | rapid, corkscrew motility | | Medium containing antibiotics against other fecal flora | extracellular |
| Legionella | | Gram-negative, but stains poorly | Slender rod in nature, cocobacillary in laboratory. monotrichious flagella | unencapsulated | | motile | | Specialized medium | facultative intracellular |
| Leptospira | | Gram-negative, but stains poorly | Long, very slender, flexible, spiral- or corkscrew-shaped rods | | | highly motile | | Specialized medium | extracellular |
| Listeria | | Gram-positive, darkly | Slender, short rods | | diplobacilli or short chains | Distinct tumbling motility in liquid medium | | enriched medium | intracellular |
| Mycobacterium | | (none) | Long, slender rods | | | nonmotile | aerobic | M. tuberculosis: Lowenstein-Jensen agar M. leprae: (none) | extracellular |
| Mycoplasma | | (none) | Plastic, pleomorphic | | singly or in pairs | | | (rarely cultured) | extracellular |
| Neisseria | | Gram-negative | Kidney bean-shaped | | diplococci | | aerobic | Thayer-Martin agar | Gonococcus: facultative intracellular N. meningitidis: extracellular |
| Pseudomonas | | Gram-negative | rods | encapsulated | | motile | Obligate aerobic | MacConkey agar | extracellular |
| Rickettsia | | Gram-negative, but stains poorly | Small, rod-like coccobacillary | | | | | (rarely cultured) | Obligate intracellular |
| Salmonella | | Gram-negative | | | | | Facultative anaerobic | MacConkey agar | Facultative intracellular |
| Shigella | | Gram-negative | rods | | | | Facultative anaerobic | Hektoen agar | extracellular |
| Staphylococcus | | Gram-positive, darkly | Round cocci | | in bunches like grapes | | Facultative anaerobic | enriched medium (broth and/or blood) | extracellular |
| Streptococcus | | Gram-positive | ovoid to spherical | | pairs or chains | nonmotile | Facultative anaerobic | blood agar | extracellular |
| Treponema | | Gram-negative, but stains poorly | Long, slender, flexible, spiral- or corkscrew-shaped rods | | | highly motile | | none | extracellular |
| Vibrio | | Gram-negative | Short, curved, rod-shaped with single polar flagellum | | | rapidly motile | Facultative anaerobic | blood- or MacConkey agar. Stimulated by NaCl | extracellular |
| Yersinia | | Gram-negative, stains bipolarly | Small rods | encapsulated | | nonmotile | nbvk | MacConkey or CIN agar | extracellular |
This is a rather clinical description of the species presented in the previous section, containing the main examples of transmission, diseases, treatment, prevention and laboratory diagnosis, which all can differ substantially among the species of the same genus.
Species of human pathogenic bacteria [8] | Species | Transmission | Diseases | Treatment | Prevention | laboratory diagnosis |
| Bacillus anthracis | - Contact with sheep, goats and horses
- Inhalation or skin penetration through abrasions of spor-contaminated dust
| | In early infection: | | - Large, grayish, nonhemolytic colonies with irregular borders on blood agar
- Direct immunofluorescence
|
| Bordetella pertussis | - Contact with respiratory droplets expelled by infected human hosts.
| Complications: | Macrolide antibiotics | | |
| Borrelia burgdorferi | Ixodes ticks reservoir in deer, mice and other rodents | | - Early stages:
- If arthritic symptoms have appeared:
- Longer courses of antibiotics
| | |
| | - Direct contact with infected animal
- Oral, by ingestion of unpasteurized milk or milk products
| | Combination therapy of: | - | |
| Campylobacter jejuni | - Fecal/oral from animals (mammals and fowl)
- Contaminated meat (especially poultry)
- Contaminated water
| | - Symptomatically by fluid and electrolyte replacement
- Ciprofloxacin in severe cases
| No available vaccine - Good hygiene
- Avoiding contaminated water
- Pasteurizing milk and milk products
- Cooking meat (especially poultry)
| - Finding campylobacter in feces
|
| Chlamydia pneumoniae | | Community-acquired respiratory infection | | None | None for routine use |
| Chlamydia psittaci | Inhalation of dust with secretions or feces from birds (e.g. parrots) | Psittacosis | | - | - Rise in antibody titer
- Complement fixation
- indirect immunofluorescence
|
| Chlamydia trachomatis | - Sexual (NGU, LGV)
- Direct or contaminated surfaces and flies (trachoma)
- Passage through birth canal (ICN)
| | | No vaccine - Erythromycin or silver nitrate in newborn's eyes
- Safe sex
| |
| Clostridium botulinum | Spores from soil and aquatic sediments contaminating vegetables, meat and fish | | | - Proper food preservation techniques
| - Mouse inoculation detects toxin from food, intestinal contents or serum
- Culture in standard aerobic culture
|
| Clostridium difficile | - Spores both indoors and outdoors
- Human flora, overgrowing when other flora is depleted
| | | None | |
| Clostridium perfringens | | | Gas gangrene: Food poisoning: - Self-limiting; Supportive care is sufficient
| Appropriate food handling | - Microscopically
- Blood agar culture, forming double-zone β-hemolysis
- Sugar fermentation
- Organic acid production
|
| Clostridium tetani | - Spores in soil infecting puncture wounds, severe burns or surgery
| | | | (difficult) |
| Corynebacterium diphtheriae | - Respiratory droplets
- Part of human flora
| | | | (no rapid) |
| Enterococcus faecalis and Enterococcus faecum | - Part of human flora, opportunistic or entering through GI tract or urinary system wounds
| | | No vaccine | - Culture in 6.5% NaCl
- Can hydrolyze esculin in presence of bile
|
| Escherichia coli (generally) | - Part of gut flora, spreading extraintestinally or proliferating in the GI tract
| | UTI: (resistance-tests are required first) Meningitis: Diarrhea: - Antibiotics above shorten duration
- Electrolyte and fluid replacement
| (no vaccine or preventive drug) - Food and water preparation
- Cooking ground beef and pasteurizing milk against O157:H7
- Hand washing and disinfection
| - Culture on MacConkey agar and study carbohydrate fermentation patterns:
- Lactose fermentation (most E. coli strains)
- Gas production in glucose fermentation
- Mannitol fermentation
|
| Enterotoxigenic Escherichia coli (ETEC) | - Fecal-oral through food and water
- Direct physical contact
| |
| Enteropathogenic E. coli | | |
| E. coli O157:H7 | | |
| Francisella tularensis | - vector-borne by anthropods
- Infected wild or domestic animals, birds or house pets
| | | - Avoiding insect vectors
- Precautions when handling wild animals or animal products
| (rarely cultured) |
| Haemophilus influenzae | - Droplet contact
- Human flora of e.g. upper respiratory tract
| | Meningitis: (resistance-tests are required first) | | |
| Helicobacter pylori | - Colonizing stomach
- Unclear person-to-person transmission
| | | (No vaccine or preventive drug) | - Microscopically
- Urease-positivity by radioactively labeled urea
- Serology by ELISA
|
| Legionella pneumophila | | | | (no vaccine or preventive drug) Heating water | - Culture from respiratory secretions on buffered charcoal yeast extract enriched with L-cysteine, iron and α-ketoglutarate
- Serology, including direct immunofluorescence and radioimmunoassay for antigen in urine
- Hybridization to ribosomal RNA using DNA probe
|
| Leptospira interrogans | - Food and water contaminated by e.g. urine from wild or domestic animals. Leptospira survives for weeks in stagnant water.
| | | (no vaccine) Prevention of exposure | |
| Listeria monocytogenes | - Dairy products, ground meats, poultry
- Vertical to newborn or fetus
| | | (no vaccine) - Proper food preparation and handling
| Isolation from e.g. blood and CSF - Beta-hemolysis and catalase production on blood agar
- Microschopy for morphology and motility
|
| Mycobacterium leprae | - Prolonged human-human contact, e.g. through exudates from skin lesions to abrasion of other person
| | Tuberculoid form: Lepromatous form: | | Tuberculoid form: - Hard to isolate (diagnosis on clinical findings and histology of biopsies)
Lepromatous form: |
| Mycobacterium tuberculosis | | | | | |
| Mycoplasma pneumoniae | - Human flora
- Droplet contact
| | | | (difficult to culture) |
| Neisseria gonorrhoeae | | | Uncomplicated gonorrhea: Ophthalmia neonatorum: | (No vaccine) | |
| Neisseria meningitidis | | | | | - Microscopy showing gram-negative diplococci, often with PMNs
- Culture on chocolate agar, giving positive oxidase test and fermentation of glucose and maltose in 5% CO2 in air
|
| Pseudomonas aeruginosa | Infects damaged tissues or people with reduced immunity. | Localized to eye, ear, skin, urinary, respiratory or gastrointestinal tract or CNS, or systemic with bacteremia, secondary pneumonia bone and joint infections, endocarditis, skin, soft tissue or CNS infections. | | (no vaccine) | |
| Rickettsia rickettsii | | | | (no preventive drug or approved vaccine) - Vector control, such as clothing
- Prompt removal of attached ticks
| |
| Salmonella typhi | Human-human - Fecal-oral through food or water
| | | - Ty21a and ViCPS vaccines
- Hygiene and food preparation
| - Isolation from blood, feces, bone marrow, urine or rose spots on skin
- Colorless, non-lactose fermenting colonies on MacConkey agar
- Serology for antibodies against O antigen
|
| Salmonella typhimurium | - Fecal-oral
- Food contaminated by fowl (e.g. eggs), pets and other animals
| | - Fluid and electrolyte replacement for severe diarrhea
- Antibiotics (in immunocompromised to prevent systemic spread)
| (No vaccine or preventive drug) - Proper sewage disposal
- Food preparation
- Good personal hygiene
| |
| Shigella sonnei | - Fecal-oral
- Flies
- Contaminated food or water
| | | - Protection of water and food supplies
- Vaccines are in trial stage[9]
| - Culture on Hektoen agar or other media for intestinal pathogens
|
| Staphylococcus aureus | - Human flora on mucosae in e.g. anterior nares and vagina, entering through wound
| Coagulase-positive staphylococcal infections: | | (no vaccine or preventive drug) - Barrier precautions, washing hands and fomite disinfection in hospitals
| |
| Staphylococcus epidermidis | Human flora in skin and anterior nares | - Infections of implanted prostheses, e.g. heart valves and catheters
| | None | |
| Staphylococcus saprophyticus | Part of normal vaginal flora | | | None |
| Streptococcus agalactiae | Human flora in vagina or urethral mucous membranes, rectum | | | None | |
| Streptococcus pneumoniae | - Respiratory droplets
- Often human flora in nasopharynx (spreading in immunocompromised)
| | | - 23-serotype vaccine for adults (PPV)
- Heptavalent conjugated vaccine for children (PCV)
| |
| Streptococcus pyogenes | - Respiratory droplets
- Direct physical contact with impetigo lesions
| | | No vaccine - Rapid antibiotic treatment helps prevent rheumatic fever
| |
| Treponema pallidum | | | | No preventive drug or vaccine - Safe sex
- Antibiotics to pregnant women at risk of transmitting congenital syphilis
| Cannot be cultured or viewed in gram-stained smear |
| Vibrio cholerae | - Contaminated water and food, especially raw seafood
| | - Fluid and electrolyte replacement
- e.g. doxycycline to shorten duration
| - Preventing fecal contamination of water supplies and food
- Adequate food preparation
| |
| Yersinia pestis | - Fleas from animals
- Ingestion of animal tissues
- Respiratory droplets
| Plague: | | | |