Susceptibility of anaerobic bacteria to beta-lactam Antibiotics
Beta-Lactam antibiotics are critical agents in the treatment of anaerobic infections. Susceptibility to these agents, however, varies widely, depending on the specific drug and organism; has not been constant over time; and differs in various geographic locations within the United States for many species. For the organisms in the Bacteroides fragilis group, the beta-lactam antibiotics with the most consistent activity are imipenem and combinations of a beta-lactam drug plus a beta-lactamase inhibitor, such as ticarcillin/clavulanate and ampicillin/sulbactam. Antibiotics with less activity include cefoxitin, piperacillin, cefotetan, and ceftizoxime. In other species of anaerobic gram-negative bacilli, beta-lactamase production is seen with increasing frequency. In vitro susceptibility of these strains is now similar to that of the B. fragilis group, with imipenem, ticarcillin/clavulanate, ampicillin/sulbactam, and cefoxitin being the most active drugs. The anaerobic gram-positive cocci and bacilli remain, for the most part, highly susceptible to penicillins and imipenem.
Antibiotic Treatment of Anaerobic Infections
The discovery of effective antimicrobial drugs has substantially improved the therapeutic outlook for patients with anaerobic infections. In addition, the pathophysiology of anaerobic infections has been elucidated by comparative antibiotic trials and the use of antibiotic probes in experimental animal models. Facultative bacteria, such as Escherichia coli, are responsible for acute peritonitis and sepsis associated with bowel perforation. Anaerobes, particularly Bacteroides fragilis
, play the seminal role in subsequent abscess formation. Treatment of only the facultative bacteria, without adequate antibiotic coverage for anaerobic bacteria, leads to clinical failures with complications of abscess formation. Such therapeutic misadventures have been witnessed in the treatment of mixed infections with cephalosporins and penicillins that lack significant activity against anaerobes. Similarly, use of metronidazole or clindamycin as a single agent is associated with failures caused by infection with facultative bacteria. Mixed infections involve complex interactions between facultative bacteria and strict anaerobes, many of which possess intrinsic pathogenicity. The best therapeutic results are realized with antimicrobial drugs that are active against both types of microorganisms.
Antibiotic Production by Anaerobic Bacteria
Soils from aerobic and anaerobic sources were investigated for the possible presence of bacteria which produce antibiotics under anaerobic conditions of growth. The screening techniques devised for this study yielded 157 soil bacteria which, during anaerobic growth, produced antibiotic activity against aerobic test bacteria.
Studies on choice of media, presence of oxygen, and changes in antibiotic activity during growth indicated that representative strains of these bacteria produced mixtures of antibiotics. The activity was heat labile.
The stability toward
heat of the antibiotics produced by these isolates
was tested by heating 10-ml aliquots of medium P-1
culture supernatant solutions, adjusted to pH 7, for 5
min in an Arnold sterilizer. The preparations were
immediately cooled in ice water after heating. As may
be seen in Table 4, all antibiotic activity in these preparations was destroyed by the heat treatment.
Source - www.ncbi.nlm.nih.gov