Anaerobic bacteria in the clinical microbiology laboratory

Recent advances in anaerobic microbiology require more sophisticated techniques and newer knowledge. Using these newer methods gives results that have not been available in routine clinical laboratories before. Interpreting the data is made difficult because of the lack of experience of most medical technologists. Many questions come up such as: What is the frequency of anaerobes? How often do the various species such as Eubacterium lenturn occur? What is the frequency of co-isolation with aerobes and other anaerobes? A review of the literature is of no real help because of the many different methods that have been used for isolation and identification. Another reason is that much of the published work has been done in research or reference laboratories and do not necessarily relate to the situation in a routine clinical laboratory. The most recent methods developed for anaerobic work include an anaerobic glove box and the roll tube apparatus. Both of these methods v/ere used to determine the frequency of isolation of the various anaerobes in clinical specimens submitted for anaerobic culture, to the Clinical Laboratory at the University of Utah Medical Center. Identification procedures included the use of a thermal conductivity gas chromatograph. A total of 114 specimens were processed. 75.4% contained at least one anaerobe. The most common specimens were from abdominal sources (44%). 81.6% of the specimens contained either aerobes or anaerobes or a combination. Of the culture positive specimens, 92.5% had anaerobes and 64.5% had aerobes. Racteroides fragilis was mixed most often with aerobic organisms (13%). Of those mixed cultures 10/21 (48%) were with Escherichia coli. Both are gram negative rods. The anaerobic gram positive cocci were in mixed culture most often with the aerobic gram positive cocci (42%). The most frequent anaerobic isolates were: B. fragilis 18%, Ps. Magnus 8%, Fusobacterium sp. 8%, Eubacterium sp, 7%, Bacteroides sp. 7%, Ps. intermed!us 6%, Ps. Anaerobius 6%, B. oralis 5%, P. acnes 5%, Bifidobacterium 5%, Veil lone!la sp. 5%, Pc. Assacharolyticus 5%, and Pc. Prevotii 5%. These data indicate that unless a primary plating method is used that will allow for separation of both aerobes and anaerobes there will be a tendency to overlook anaerobes because of the frequency of co-isolation with aerobic organisms that have the same general gram morphology. The same reasoning holds true for the gram stain of the original specimen. If a gram negative rod was seen on the original smear and E. coli was isolated on the aerobic plates the assumption that all organisms present in the specimen v/ere isolated does not always hold true.