This study explored the use of D-lactic acid as a marker for bacterial infections. D-Lactic was produced by frequently encountered human bacterial pathogens under anaerobic growth conditions; Bacteroides fragilis produced the largest amount. Orally administered D-lactic acid was absorbed from the intestines of rats and later found in measurable quantities in the blood and urine. Eunephric and anephric rats that received D-lactic acid intravenously showed similar quantities of this metabolite in the blood. These quantities are consistent with the distribution of D-lactic acid to total body water. Isolated liver and lung tissues from rats did not metabolize or produce D-lactic acid. Rats with experimentally induced, sublethal klebsiella peritonitis had D-lactic acidemia of 0.2 mM and 25.6 mM at 0 and 6 hr of infection, respectively. In a normal human, D-lactic acid was detected in the urine and blood after a subcutaneous injection of D-lactic acid, and pharmacokinetics of elimination similar to those of rats were found.

Seven ,-lactam antibiotics were studied for both their antimicrobial activity and the degree to which they produced inoculum effect on Escherichia coli, Klebsiella pneumoniae, and Salmonella typhimurium. Aztreonam, cefoperazone, and ceftazidime were poorly bactericidal, caused marked bacterial filamentation, and exhibited a large inoculum effect on E. coli, K. pneumoniae, and S. typhimurium. Cefotaxime and ceftriaxone were more rapidly bactericidal, caused only a moderate amount of filamentous forms, and exhibited a modest inoculum effect, while cefoxitin and imipenem both were rapidly bactericidal and exhibited only a minimal-to-noinoculum effect. The inoculum effect did not correlate with drug stability during incubation with the bacteria. Inoculum effect on these species of the family Enterobacteriaceae appears to be a manifestation of increase in optical density secondary to the development of filamentous bacterial forms with an increase in bacterial mass during exposure to antibiotics which are not rapidly bactericidal. These observations have a clear significance for the susceptibility testing of beta-lactam antibiotics when turbidity is used as a parameter to determine presence of bacterial growth.Although the clinical importance of the inoculum effect of antibiotics is as yet unclear, it is of major importance in laboratory susceptibility testing (2, 4). The inoculum effect, or the dependence of susceptibility results on the inoculum size, has been shown to be particularly large and dramatic for Pseiudoinoo(is aerllginosa( with the f3-lactam antibiotics (4, 5). In that setting, those P-lactam antibiotics which did not rapidly kill the organisms but caused the organisms to form filamentous structures were shown to have large inoculum effects (5). The explanation for the presence and the extent of the inoculum effect is not known for the P-lactam antibiotics with members of the family Enterobacteriaceae, which are generally regarded as very susceptible to these antibiotics (1).The following experiments were designed to determine the relationship between inoculum effect and the morphological changes induced in the gram-negative rods by the P-lactam antibiotics. We examined the ability of a variety of betalactam antibiotics to kill bacteria, to induce filamentous forms, and to exhibit an inoculum effect on Esclerichia co/i, Klebsiella pneintoniaie, and Salmoiel(l typhimnmmrimn. MATERIALS AND METHODSIsolates and antibiotics. One clinical isolate each of E. coli, K. pneirmoniae, and Stilmonella tvphimnmmiuiin was studied. These organisms were identified by the API system (Analytab Products, Plainview, N.Y.), conventional biochemical studies, and serological agglutination (salmonella). The isolates of E. coli and Salmoniella tvplhimnriaum had typical susceptibility patterns and were susceptible to ampicillin, and K. Inoculum effect. The degree of the inoculum effect of each antibiotic on the three isolates was tested by both microbroth (7) and macrobroth (7) dilution with inocula of 5 x 1(5 and 5 x 107 CFU/ml. ...

Rifampicin resistance developed easily in methicillin-susceptible and methicillin-resistant strains of Staphylococcus aureus during an overnight incubation in broth containing 0.1 mg/l of rifampicin. Incubation of methicillin-susceptible Staph. aureus and 0.1 mg/l of rifampicin with 1 mg/l of nafcillin reduced the emergence of rifampicin resistance with only 5 of 50 strains (10%) becoming rifampicin-resistant. However, incubation of the methicillin-susceptible or methicillin-resistant strains with 0.1 mg/l of rifampicin and 1 mg/l of vancomycin did not prevent the development of rifampicin resistance. Rifampicin resistance developed in 25 of 50 (50%) of methicillin-susceptible and 32 of 50 (64%) methicillin-resistant Staph. aureus strains tested. These data would suggest that differences exist in the abilities of nafcillin and vancomycin to suppress the development of rifampicin resistance in Staph. aureus (P less than 0.01). Caution should be exercised when the combination of vancomycin and rifampicin is used for infections caused by Staph. aureus and Staph. aureus isolates recovered during therapy should be monitored for the development of rifampicin resistance.

One Salmonella and four Escherichia coli isolates from patients with bacterial meningitis who had responded slowly, relapsed, or failed to respond to monotherapy with moxalactam were examined. For purposes of comparison, an E. coli isolate from one patient who had responded promptly to therapy was also studied. On testing, moxalactam had higher MICs and MBCs (two to four times) than cefotaxime or ceftriaxone for all isolates; the rates of killing of the isolates were dependent on the antibiotic concentrations used. At comparable multiples of the MIC, these isolates were generally killed more slowly by moxalactam than by cefotaxime or ceftriaxone. In addition, a reduction of 3 in the logarithm of the number of CFU per milliliter could be attained at far lower concentrations with cefotaxime or ceftriaxone than with moxalactam. The degree of concentrationrelated killing of bacteria produced by the beta-lactams appeared to correlate with the clinical responses of the patients. Furthermore, real differences appeared to exist among the third-generation cephalosporins, which were not evident by the MIC and MBC points alone but were evident in the concentration-related killing curves. Determination of a reduction of 3 in the logarithm of the number of CFU per milliliter after a 6-h incubation is suggested as the criterion for the screening of antibiotics for the therapy of gram-negative bacillary meningitis.Soon after their introduction, the newer third-generation cephalosporins were generally accepted as potential antibacterial agents of choice for the therapy of gram-negative bacillary meningitis (1,4,11). A recent survey of infectious disease consultants in the United States showed that these antibiotics were indeed being used for the treatment of meningitis cases involving a wide range of bacterial pathogens (C. E. Cherubin, manuscript in preparation).In marked contrast to previously published series, scattered reports of therapeutic failures have begun to appear in the medical literature (2,5,8,12 Susceptibility testing. The CSF isolates were tested for susceptibility to antibiotics in three ways. First, the isolates were tested by the macro-broth dilution method with an inoculum of 5 x 105 CFU in 1 ml (14). A 0.1-ml portion was subcultured from each tube after an 18-h incubation at 35°C, and the criterion of 99.9% killing was used for the MBC. Second, the method described by Eng et al. (6) was used to ascertain the effect of a large inoculum on the susceptibility results. For this procedure, the organisms were tested by the same macro-broth dilution method but at an inoculum of 5 x 107 CFU/ml. The MIC at the higher inoculum was determined by the highest drug dilution which inhibited turbidity formation in the tube, compared with a control tube containing 1% Formalin and the same inoculum (6). A 100-pl sample from each tube was subcultured after an 18-h incubation at 35°C, and the same criterion of 99.9% killing was used to determine the MBC. Third, the organisms were retested in CSF and in Mueller-Hinton broth by ...