Chapter Checkpoints
These are some important ideas you are learning in Chapter 20:

Medically Important Gram-Negative Bacilli
Medically significant gram-negative bacteria are divided into three groups based on their oxygen requirements. Pathogenic species are aerobic or facultative anaerobes. Opportunist pathogens occur in all three groups. One common virulence factor in all gram-negative pathogens is endotoxin, the LPS component of the cell wall. Endotoxin stimulates excessive release of host defense mediators which, in turn, result in high fever, tissue damage, hypotension, and shock.

Aerobic gram-negative nonenteric bacilli
Medically important gram-negative aerobes include the opportunistic pathogen Pseudomonas, the zoonotic pathogens Brucella and Francisella, as well as the human pathogens Bordetella and Legionella. Pseudomonads normally inhabit soil and water. They are metabolically quite versatile, able to degrade many substances, and survive under minimal conditions.

Ps. aeruginosa is a common nosocomial opportunist, infecting human hosts that are either debilitated or have had invasive medical procedures. Infections occur primarily in patients with severe burns, neoplastic disease, or cystic fibrosis. Healthy individuals acquire skin infections from swimming pools, hot tubs, contaminated sponges, or contact lens solutions.

Brucella infections occur primarily through exposure to infected cattle or pigs, but also through drinking unpasteurized milk. Brucellosis is a systemic infection characterized by alternating periods of fever, sweating, and chills. The infection is carried by neutrophils to many body organs. Combination drug therapy is effective. Prevention includes animal quarantine and vaccination.

Francisella tularensis is the causative agent of tularemia, an arthropod-borne zoonosis infecting rabbits, rodents, and other mammals. Infection occurs through skin, eye, G-I tract, and respiratory tract. Tularemia is highly transmissible through direct contact with infected animals, infected water, droplets, dust, or through the bites of infected arthropods, but not direct contact with other humans. Streptomycin is effective in most cases. Immunization provides protection for occupational risk groups.

Bordetella pertussis is the causative agent of whooping cough, or pertussis, a potentially fatal respiratory infection of infants transmitted through droplets. Treatment with erythromycin is usually effective. Childhood immunization provides protection.

Legionella is the causative agent of legionellosis (Legionnaires' disease) and the milder Pontiac fever. Both are non-communicable lung infections transmitted by aerosols from air conditioners, cooling towers, and other aquatic habitats. Legionella exists in aquatic habitats, possibly in symbiosis with certain amebas. Legionellosis is a potentially fatal disease if untreated, but it responds to antibiotic therapy.

Identification and Differential Characteristics of the Enterobacteriaceae
Enterobacteriaceae are a large family of gram-negative rods. Some live independently, but many live as enterics, the normal flora of vertebrate intestinal tracts. Enteric species are grouped as coliforms or noncoliforms, based on their ability to ferment lactose. Enteric pathogens are identified by growth characteristics on differential media and their reactions to the IMVIC test. Virulence factors of pathogenic species include enterotoxins, exotoxins, capsule, fimbriae, hemolysins, and endotoxin.

Pathogenic E. coli infections are usually limited to the intestine or urinary tract. An exception is E. coli 0157:H7 which causes hemolytic anemia and kidney damage. Virulence factors are two exotoxins, which cause excessive fluid loss, plus fimbriae that enable it to adhere to the intestinal mucosa. The presence of E. coli in food, water, dairy products indicates fecal contamination, and therefore the possible presence of enteric pathogens.

Enteric species Klebsiella, Enterobacter, Serratia, and Citrobacter are not usually harmful to the healthy host, but can cause serious, even fatal, nosocomial infections when introduced into non-intestinal sites. Klebsiella is the most important secondary invader in this group.

The noncoliform enterics Proteus, Morganella, and Providencia are also opportunist pathogens which are often involve in septicemia, wound infection, urinary tract infections, and pneumonia.

Salmonella and Shigella are noncoliform pathogens that cause intestinal infections with a wide range of severity, depending on the species. Salmonella is a hardy genus that survives inhospitable environments. It infects animals as well as humans. Salmonella is transmitted both directly and also indirectly through contaminated food and water. S. typhi causes the most serious disease to typhoid fever. Other species cause salmonella food poisoning, gastroenteritis, and enteric fevers. Humans are the only host for S. typhi, but other Salmonella pathogens are transmitted through poultry and dairy products, human feces, and other fomites.

Shigellosis, or bacillary dysentary, is a disease of worldwide distribution. It is caused by several species of Shigella. Infections are easily initiated because of the small infective dose. Humans are the only reservoir. Virulence factors are endotoxin and enterotoxins which attack the colon.

Yersinia species are agents of zoonotic infections. Y. enterocolitica and Y. pseudotuberculosis cause enteric infections, primarily in children. They are transmitted through contaminated food and water. Y. pestis causes plague in three forms: bubonic, pneumonic, and septicemic. It is usually transmitted to humans through flea bites. Mice and rodents are the primary reservoirs of this bacillus, but infected fleas spread it to other mammals (amplifying hosts) and from there to humans. Fleas remain infected for up to three years. Infections respond to antibiotic therapy if treated early, but untreated infections have high mortality rates.

Pasteurella is another genus that causes a wide range of opportunistic zoonoses. P. multocida occasionally causes infection in humans through animal bites or scratches .

The genus Haemophilus contains both commensals and pathogenic species. Young children and the elderly are most susceptible to infections. Haemophilus influenzae causes a variety of upper and lower respiratory infections. Haemophilus meningitis, or acute bacterial meningitis, is the most serious infection. Its symptoms are similar to Neisseria meningitis, but it does not spread through populations as readily. Haemophilus infections respond to combination drug therapy. Immunization is available for young children. Other pathogens include Haemophilus aegypticus, the causative agent of conjunctivitis, H. ducreyi, the agent of soft chancre, an STD, and H. parainfluenzae and H. aphrophilus, agents of bacterial endocarditis.

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