ENTERO BACTERIACEAE : CLINICAL ASPECTS
The Enterobacteriaceae produce the widest variety of infections of any group of microbial agents, including two of the most common infectious states, UTI and acute diarrhea. UTIs are manifested by dysuria and urinary frequency when infection is limited to the bladder, with the addition of fever and flank pain when the infection spreads to the kid-ney. Enterobacteriaceae are by far the most common cause of UTIs, and the most com-mon species involved is E. coli.
Culture is the primary method of diagnosis; all Enterobacteriaceae are readily isolated on routine media under almost any incubation conditions. Special indicator media such as MacConkey agar are commonly used in primary isolation to speed separation of the many species. For example, the common pathogens E. coli and Klebsiellatypically ferment lac-tose rapidly, producing acid (pink) colonies on MacConkey agar, whereas the intestinal pathogens Salmonella and Shigella do not. Separation of the intestinal pathogens from all the other Enterobacteriaceae present in stool requires the use of highly selective media de-signed solely for this purpose. They will be discussed as they relate to individual pathogens.
Improved understanding of the genetic and molecular basis for virulence has led to the development of direct nucleic acid and immunodiagnostic techniques for direct detec-tion of toxin, adhesin, or invasin genes in clinical material (eg, stool). These methods are still too expensive for use in clinical laboratories but are of extraordinary value in epi-demiologic work and clinical research.
Antimicrobial therapy is crucial to the outcome of infections with members of the Enter-obacteriaceae. Unfortunately, combinations of chromosomal and plasmid-determined re-sistance render them the most variable of all bacteria in susceptibility to antimicrobial agents. They are usually resistant to high concentrations of penicillin G, erythromycin, and clindamycin, but may be susceptible to the broader-spectrum -lac-tams, aminoglycosides, tetracycline, chloramphenicol, sulfonamides, quinolones, nitrofu-rantoin, and the polypeptide antibiotics. Because the probability of resistance varies among genera and in different epidemiologic settings, the susceptibility of any individual strain must be determined by in vitro tests. Typical frequencies of resistance for some of the more common Enterobacteriaceae appear in Table 13 – 1.