Citron, D.M.,
R.M. Alden Research Laboratory, Santa Monica, CA. USA

The increasing use of molecular methods has resulted in an increase in new genus and species names that are important in the research setting to characterize the pathogenesis and antibiotic susceptibility of anaerobes. However, in the clinical setting where patient care is of primary importance, a rapid report of the presence of anaerobic bacteria with a description of a group or genus will influence patient care more than a detailed report of esoteric names that arrives weeks after the specimen was collected. Rapid presumptive identification of the B. fragilis group, certain clostridia, or simply a report of the presence of anaerobes with a description of their appearance on gram-stain provide guidelines to the clinician for antimicrobial therapy or surgical intervention. In the case of bacteremia, this information may suggest the location of the primary infection. The use of selective and differential agars for the primary set-up of specimens and prompt incubation in an anaerobic environment allow for recognition of distinctive characteristics such as pigment, hemolysis, or fusiform cells on gram-stain, which can provide presumptive group or genus level identification within 24-48h of receipt of the specimen. Examples of these agars include Bacteroides-bile-esculin (BBE) agar for isolation and presumptive identification of the Bacteroides fragilis group and Bilophila, and LKV agar for Bacteroides, Prevotella and some strains of fusobacteria. PEA and CNA blood agars inhibit enteric organisms but allow for growth and isolation of anaerobic gram-positive and gram- negative anaerobes. Simple tests, such as susceptibility to the 1000mg kanamycin disk, spot indole, catalase, nitrate reduction, and urease production are rapid and useful for initial grouping of anaerobes. Further identification is often unnecessary. Complete identification is desirable for anaerobes recovered from blood culture, pure cultures from any site, and in instances where prolonged antimicrobial therapy is anticipated, such as for undrainable abscesses, chronic osteomyelitis, and other serious infections. When used in conjunction with initial observations, tests for preformed enzymes are convenient and help to identify the majority of clinically important anaerobes. Preformed enzyme tests are preferable to conventional biochemical tests because they do not require growth of the organism and can be interpreted after 1 to 4 hours of incubation. Judicious choices can provide clinically relevant information promptly and inexpensively.