Chapter Capsule

The gram-negative rods are a large group of loosely affiliated families and genera, all of which are non-spore-forming; cell walls of most species contain lipopolysaccharide with endotoxin, which causes fever, cardiovascular disruptions, and shock; gram-negative septicemia is a dangerous complication of infection.

I. Aerobic Rods

A. Pseudomonas species are among the most widely distributed bacteria; thrive even in hostile conditions or where nutrients are scarce; medically, Ps. aeruginosa is a common opportunist of medical treatments when normal defenses are compromised; may attack lungs, skin, urinary tract, eyes, ears, hosts with severe burns; may also infect healthy persons; drug resistance limits treatment choices.
B. Brucella is a zoonotic genus that causes abortion in cattle, pigs, and goats, and brucellosis, or undulant fever, in humans.
1. Bacterium is transmitted through direct contact with infected animals or contaminated animal products and ingestion of raw milk; infection occurs in several systems.
2. Treated with streptomycin and rifampin; controlled through animal vaccination, pasteurization.
C. Francisella tularensis causes tularemia, or “rabbit fever,” a zoonosis of rabbits, rodents, and other wild mammals that spreads to humans through direct contact with animals, bites by vectors (ticks), ingestion of contaminated food or water, or by inhalation.
1. Symptoms occur in skin, lymph nodes, lungs, intestine.
2. Treated with gentamicin; vaccine available for risk groups.
D. Bordetella pertussis causes a strictly human disease called pertussis, or whooping cough; contagious and prevalent in children under 6 months; infectious by droplets; attachment of pathogen and toxin production destroy cilial defense and produce a series of coughs that occur in bursts followed by a sudden inspiration; vaccine (acellular pertussis) is a very effective preventive.
E. Legionella pneumophila causes legionellosis, commonly called Legionnaires disease; usual agent is a wide-ranging inhabitant of natural water that survives for months in artificial aquatic environments (cooling towers, air conditioners, taps) and can cause serious lung disease when inhaled.
1. Community infections occur from common environmental sources.
2. Hospital epidemics occur due to contaminated air and water supplies.

II. Facultative Anaerobic Rods

The Family Enterobacteriaceae is the largest group of gram-negative enteric bacteria; small, often motile, fermentative rods occurring in many habitats but often found in animal intestines; informal division of family is made between coliforms, normal flora that are rapid lactose fermenters, and noncoliforms, genera that do not or only weakly ferment lactose; enterics are predominant bacteria in clinical specimens; some species cause diarrheal disease due to enterotoxins acting on the intestinal mucosa or to invasion and disruption of the mucosa; identification of group includes series of biochemical and serological tests; primary antigens are flagellar (H), cell wall (O), and capsular (K or V_i); drug resistance is well entrenched through plasmids; treatment with drugs requires sensitivity testing.

A. Escherichia coli exists in several forms; pathogenic strains have acquired virulence factors for invasiveness and toxigenicity; cause of infantile diarrhea, a complication of malnourished babies fed unsanitary food or water; traveler’s diarrhea occurs in people who pick up a toxigenic strain from water and food in other countries. E. coli is the usual cause of urinary tract infections (from normal flora) and nosocomial pneumonia and septicemia.
B. E. coli 0157:H7 is an emerging pathogen of cattle and humans; acquired primarily through beef, produce, and water; causes hemolytic uremic syndrome due to kidney damage.
C. Other coliforms are ubiquitous in the hospital environment. Klebsiella, Enterobacter, Serratia, and Citrobacter account for nosocomial infections associated with tracheostomies, respiratory care equipment, endoscopes, and catheters; other common infections are pneumonia and burn and incision infections.
D. Noncoliform infections are caused by opportunists, pathogenic enterics, and pathogenic nonenterics. Opportunists include Proteus, Morganella, and Providencia, which cause nosocomial infections such as urinary tract infections, wound infections, pneumonia, sepsis, and diarrhea.

III. True enteric pathogens include Salmonella and Shigella.

A. Salmonella causes salmonelloses; most severe disease is typhoid fever, caused by S. typhi; agent is spread only by humans via unclean food or water.
1. The bacillus crosses wall of small intestine into circulation, is carried to organs, where it may form abscesses.
2. Symptoms include fever, diarrhea, septicemia, and ulceration of small intestine; perforation is a complication.
3. Treatment with chloramphenicol; vaccine available.
4. Other species belong to a serotype of S. enteritidis, such as S. typhimurium or S. paratyphi A; bacilli are common flora of animals; contaminate meat, milk, and eggs; disease somewhat milder but more prevalent than typhoid fever; main infections are enteric fever or gastroenteritis, depending on severity of symptoms; treatment with antibiotics, oral rehydration.
B. Shigella causes shigellosis, a bacillary dysentery characterized by acute painful diarrhea with bloody, mucus-filled stools; species are Sh. dysenteriae, Sh. sonnei, and Sh. flexneri; all are primarily human parasites; spread by fingers, feces, food, and flies; bacteria remain local to large intestine; damage to intestinal villi causes symptoms; treated with oral antimicrobics and rehydration therapy. Enteric disease is preventable through cleanliness in processing food, adequate cooking and refrigeration, awareness of animal carriers, proper toilet habits, control of water and sewage, monitoring carriers, and avoiding questionable food or water.

IV. True pathogens in Yersinia, Pasteurella, and Haemophilus genera.

A. Yersinia causes yersinioses, zoonoses spread from mammals to humans; Y. enterocolitica and Y. pseudotuberculosis cause food infection with appendicitis-like symptoms. Yersinia pestis is a nonenteric agent of the plague, an ancient virulent disease with complex epidemiology; agent is maintained by relationship between endemic hosts (mice) and amplifying hosts (rats, squirrels) and flea vectors that carry the bacillus between them; humans enter this cycle through flea bite or contact with infected animal; infected humans may pass the agent to other humans. Forms are:
1. Bubonic plague, in which multiplication at site of bite creates regional lymphatic swelling, or bubo.
2. Septicemic plague, a deadly complication marked by hemorrhage.
3. Pneumonic plague, a lung infection spread by aerosols. Treatment includes streptomycin, tetracycline; vector and reservoir control and a vaccine can be preventive.
B. Nonenteric pathogens include:
1. Pasteurella multocida, a zoonosis of cattle, poultry, cats, dogs; spread by bites, scratches, other contact; usual disease is abscess and lymph node swelling.
2. Haemophilus influenzae is the most frequent cause of acute bacterial meningitis in children between 3 months and 5 years; sporadic infection occurs primarily in daycare and similar settings; passed through respiratory discharges; disease is marked by acute neurological complications, high morbidity, and sequelae; treated with chloramphenicol and ampicillin; vaccination with Hib recommended for children at risk.
3. H. aegyptius causes a form of conjunctivitis called pinkeye.
4. H. ducreyi causes the STD known as chancroid.

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