Bacterial Infections

E coli is the most common member of the Enterobacteriaceae to be isolated in the clinical microbiology laboratory. Most E coli isolates are either opportunistic or nosocomial pathogens (ie, causes of urinary tract or wound infections) or normal flora (ie, enteric contaminants of urine cultures or normal stool flora in stool cultures).

N meningitidis is found only in humans and is a member of the normal oropharyngeal flora in 5-15% of healthy adults and children. In crowded or closed populations such as in boarding schools or military camps, higher carriage rates are observed.

Neisseria gonorrhoeae was first described by Albert Neisser in 1879, in the ocular discharge and exudate from newborn infants with conjunctivitis. Descriptions of a condition resembling the disease gonorrhea can be found in the written record as early as 130 AD, when Galen created a descriptor for the malady by using the Greek words gonos (seed) and rhoea (flow) to characterize what was believed to be the morbid loss of semen.

L monocytogenes is found in soil, fertilizer, sewage, and stream water; on plants; and in the intestinal tracts of many mammals. It is a foodborne pathogen that causes bacteremic illness and meningoencephalitis, with few if any gastrointestinal manifestations.

Humans are the only known natural hosts for C diphtheriae, the organism that causes diphtheria. This organism is usually spread via upper respiratory tract droplets, but it can also be spread by direct contact with skin lesions. Transmission appears to be more common when people are living indoors in crowded conditions.

Bacillus species other than B anthracis are found in soil, decaying organic matter, and water, but they are rare causes of disease. Risk factors associated with Bacillus infection include the presence of intravascular catheters, intravenous drug use, sickle cell disease, and immunosuppression — particularly corticosteroid use, transplantation, AIDS, and neutropenia secondary to chemotherapy.

Historically, anthrax has been an occupational disease of persons who handle animal hair, skin, and other contaminated products. The incidence of this disease in the United States has fallen dramatically; only six cases of anthrax were reported to the Centers for Disease Control and Prevention from 1978 through 1998. The cutaneous form of the disease is most common.

The symptoms of pharyngitis caused by these organisms mimic those of S pyogenes pharyngitis (Box 50-1; see also site). Poststreptococcal glomerulonephritis has been described following S dysgalactiae subspp. equisimilis and S zooepidemicus pharyngitis.

Early-onset group B streptococcal neonatal infection has three major clinical expressions: bacteremia with no identifiable focus of infection, pneumonia, and meningitis (Box 1). Signs and symptoms of early-onset group B streptococcal neonatal infection include lethargy, poor feeding, jaundice, abnormal temperature, grunting respirations, pallor, and hypotension.

Viridans streptococci are part of the normal microbial flora of humans and animals and are indigenous to the upper respiratory tract, the female genital tract, all regions of the gastrointestinal tract, and, most significantly, the oral cavity. Clinically significant species that are currently recognized as belonging to the viridans group of streptococci include Streptococcus anginosus S constellatus, S cristatus, S gordonii, S intermedius, S oralis, S mitis, S mutans, S cricettus, S rattis, S parasanguis, S salivarius, S thermophilus, S sanguinis, S sobrinus, and S vestibularis. Detailed studies of the ecology of strains in the oral cavity and oropharynx have been performed.