Synergism between penicillin, clindamycin, or metronidazole and gentamicin against species of the Bacteroides melaninogenicus and Bacteroides fragilis groups

Treatment of anaerobic infection

Anaerobic bacteria are the predominant flora in the normal human skin and mucous membranes and are, therefore, a common cause of endogenous infections. Since anaerobic infections are generally polymicrobial, where anaerobes are mixed with aerobic organisms, therapy should provide coverage of both types of pathogens. The isolation of anaerobes requires appropriate methods of collection, transportation and cultivation of specimens. The lack of use of any of these methods can lead to inadequate recovery of anaerobes and inappropriate therapy. Treatment of anaerobic infection is complicated by the slow growth of these organisms and the growing resistance of anaerobic bacteria to antimicrobials. The primary role of antimicrobials is to limit the local and systemic spread of infection. Surgical drainage is of primary importance. This includes debriding of necrotic tissue, draining the pus, improving circulation, alleviating obstruction and increasing tissue oxygenation. The most effective antimicrobials against anaerobic organisms are metronidazole, the carbapenems (imipenem, meropenem and ertapenem), chloramphenicol, the combinations of a penicillin and a beta-lactamase inhibitor (ampicillin or ticarcillin plus clavulanate, amoxicillin plus sulbactam, and piperacillin plus tazobactam), tigecycline and clindamycin.

In vitro cultivation and antibiotic susceptibility of a Cytophaga-like intracellular symbiote isolated from the tick Ixodes scapularis

A Cytophaga-like organism (CLO), isolated from the tick Ixodes scapularis (IsCLO), was adapted to growth in insect cell lines and its antibiotic sensitivity was tested. IsCLO were introduced to four insect cell lines, and their growth was measured by quantitative polymerase chain reaction. IsCLO propagated well in a mosquito cell line, AeAl-2, and caused cytopathic effects in host cells. A lepidopteran cell line, HZ-AM1, was also suitable for propagation of IsCLO and kept a steady state with bacterial growth. Using IsCLO-infected AeAl-2, antibiotics effective against the bacteria included ampicillin, chloramphenicol, penicillin-G, rifampicin, and tetracycline. These antibiotics will be useful for eliminating CLO from host arthropods, which is necessary for in vivo studies of the intracellular facultative symbiotes.

The effect of antimicrobial therapy on mixed infections with Bacteroides species. Is eradication of the anaerobes important?

Antimicrobial agents were used alone or in combinations in order to explore their effect on mixed aerobic-anaerobic infections. Subcutaneous abscesses were induced in mice by single and mixed infections of Bacteroides fragilis, Bacteroides melaninogenicus, Staphylococcus aureus, Streptococcus pyogenes, Streptococcus faecalis, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. The infected animals were treated for 5 days with spiramycin, gentamicin or metronidazole alone, or metronidazole combined with spiramycin or gentamicin. Animals were killed 5 days after inoculation and the bacterial contents of the abscesses determined. Infection induced by a single species of bacteria always responded to appropriate antimicrobial therapy. In infections caused by two species of organisms, however, therapy directed at either the Bacteroides sp. (with metronidazole) or the aerobes or facultative anaerobes (with spiramycin or gentamicin) was effective not only in significantly reducing the numbers of the target organism but also, in 13 of 24 instances, in reducing to a small extent the numbers of the other bacteria. Despite this phenomenon, in no instance did therapy with a single agent eliminate the infection and eradicate the untargeted organism. The combination of spiramycin and metronidazole increased the reduction in numbers of B. melaninogenicus in single-organism infections and of Bacteroides sp. in mixed infections with S. aureus and S. pyogenes. These findings support the need to aim treatment at all components of mixed infections.

Management of infection following intra-abdominal trauma

Intra-abdominal infections following abdominal trauma often involve the gastrointestinal aerobic and anaerobic bacterial flora. These organisms possess various virulence factors and exhibit potential synergy between them. The intra-abdominal infection is biphasic, with the Enterobacteriaceae as the major pathogens in the peritonitis stage, and the Bacteroides fragilis group predominant in the abscess stage. Experiments with animals and experience in human beings support the need to use single or combined antimicrobial agent therapy that is effective against both Enterobacteriaceae and the B fragilis group.

Bacterial synergy in pelvic inflammatory disease

Polymicrobial aerobic and anaerobic flora are responsible for pelvic inflammatory disease (PID). The most frequent pathogens appear to be Neisseria gonorrhoea and anaerobic bacteria (most commonly anaerobic cocci and Bacteroides sp.). Recent studies have demonstrated the recovery of Chlamydia trachomatis in up to a third of these infections. Although N. gonorrhoea is frequently recovered from cervical cultures, it is less commonly recovered from intra-abdominal sites. Recent studies have demonstrated the in-vivo synergistic relationship between N. gonorrhoeae and Bacteroides fragilis. The growth of each component of the mixed infection was enhanced when these were present together in an abscess. Furthermore, the emergence of encapsulated strains was enhanced in these infections. This synergy enables the organisms to cause more severe local and systemic damage to the host. Therapeutic intervention should include the use of antimicrobial agents effective against both the aerobic and anaerobic components of the mixed infection. Unless such therapy is given, the infection may persist. It is also recommended to use antimicrobials that possess synergy between them against the bacterial pathogens. Agents effective against the anaerobic pathogens are metronidazole, clindamycin and cefoxitin. Antimicrobials effective against the Gram-positive aerobic pathogens and N. gonorrhoeae are spiramycin and penicillins. Aminoglycosides or third generation cephalosporins are effective against Gram-negative enterics. The combination of metronidazole and spiramycin has shown to be synergistic against mixed infections of Bacteroides sp. and N. gonorrhoeae.

Pathogenicity of Clostridium species with other bacteria in mixed infections

The relationship of clostridial isolates with other bacteria in mixed infections was studied by means of a subcutaneous abscess model in mice. We used 26 isolates of seven clostridial species, two Bacteroides spp., eight Gram-positive facultative or anaerobic cocci and three enteric Gram-negative aerobic rods. Abscesses were induced by all seven Clostridium perfringens and three C. butyricum isolates and by some of the others. Selective antimicrobial therapy experiments showed that enteric Gram-negative rods were of equal or greater significance in the formation of abscesses than were clostridial strains in mixed infections. Enhancement or suppression of each component of the mixed infection was studied by comparing the number of each bacterium to its number when injected alone. Enhancement was observed mainly with C. perfringens in mixed infections. By contrast, other Clostridium spp. were less able to induce enhancement. Clostridium difficile and C. sporogenes often inhibited other bacterial species. This study demonstrated the synergistic and antagonistic relationship between clostridial species and other bacteria.

Serum and crevicular fluid concentrations after a single oral dose of metronidazole

Previous studies have shown that metronidazole is an effective chemotherapeutic agent in the treatment of certain types of periodontal disease. The purpose of this study was to assess, over 18 hours, the concentration of the drug in serum and gingival crevicular fluid after a single oral dose. Six female volunteers with gingivitis created by cessation of brushing for 2 weeks, took 250 mg of metronidazole orally. Micropipettes were used to collect 20 microliters of serum and 4 to 5 microliters of gingival fluid hourly for 8 hours, and at the 12th and 18th hours. Samples were assayed with a high pressure liquid chromatograph. Mean drug levels in serum closely matched those reported by Stephen et al. (Br Dent J 7: 313, 1966) with polography. Mean serum drug levels peaked at 6.09 micrograms/ml at the 2nd hour, and mean gingival crevicular fluid drug levels peaked at 3.62 micrograms/ml at the 2nd and 7th hours. The drug was detectable in both fluids for up to 18 hours. Mean serum concentrations remained greater than mean gingival fluid concentrations at all time intervals, though the differences were not significant (P less than 0.05) as determined by a Hoteling's T2 test. Using reported minimal inhibitory concentration values of metronidazole for various periodontopathogens, it was concluded that a single oral dose of metronidazole will deliver potentially inhibitory levels of the drug to the periodontium in serum and in gingival crevicular fluid.