Comparison of tube dilution and microtitre methods for the detection of antibiotic tolerance in strains of Staphylococcus aureus.

Comparison of the MICs and MBCs of oxacillin for ten isolates of Staphylococcus aureus determined by tube dilution with those by microtitre dilution tests demonstrated that, whereas the MICs did not differ, the mean MBC by tube dilution was significantly higher (P less than 0.05) than that by microtitre after both 24 and 48 h of incubation. In addition, tolerance (MBC/MIC ratio greater than 32) was seen in four of ten strains with the tube dilution method but in only one strain with the microtitre technique after 24 h of incubation. These results and the poor correlation (r = 0.33) between MBCs by these methods indicate the importance of technical factors in the quantitation of MBCs and resulting identification of tolerance in Staph. aureus, and the need for a reference procedure in determination of MBCs.

Limited protective effect of rough mutant antisera in murine Escherichia coli bacteremia.

Previous studies in mice have demonstrated differing immunoprophylactic activity of antisera against rough mutants of Enterobacteriaceae in the prevention of lethal gram-negative bacteremia. In this study, in which CF1 mice were made bacteremic with a serum-resistant Escherichia coli 06:K2:H1, the composite survival was significantly (p less than 0.001) enhanced by i. v. pre-treatment one to two hours before injection with either normal rabbit sera or antisera to the J5 mutant of E. coli 0111. The protective efficacy of these preimmune and hyperimmune sera did not differ significantly. Since considerable variability in the mortality of control mice occurred in the 25 separate experiments, the results of individual experiments were grouped retrospectively according to survival in the individual control groups and compared for evidence of possible differences in the efficacy of these two sera. With the exception of a statistically significant difference in the efficacy in one group receiving an LD75-95 inoculum, no such differences were noted. Thus, the variable effects of a rough mutant antiserum were not explained by differences in the relative virulence in the inoculum. This study confirms earlier observations by others that the protective efficacy of the anti-J5 antisera in infected mice does not differ appreciably from that of normal rabbit sera, provided the same donor rabbits are the source of both preimmune and hyperimmune sera.