- Volume 29, Issue 4, 1989

Eight of 22 non-capsulate strains of Neisseria meningitidis previously isolated from primary school children were re-identified as N. polysaccharea by aminopeptidase reactions and polysaccharide production. N. polysaccharea was not identified amongst 91 non-capsulate strains of N. meningitidis isolated from adults attending the Genito-urinary Medicine clinic, Westminster Hospital, London. The biochemical reactions of N. polysaccharea strains were similar to those of N. lactamica and N. gonorrhoeae, but N. polysaccharea could be distinguished from these organisms by examination of β-galactosidase activity, carbohydrate reactions and polysaccharide production. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis revealed closer similarity of N. polysaccharea to N. lactamica than to the pathogenic Neisseria spp. An additional finding was variation in the position of one of the major proteins of N. lactamica in the 34-39-Kda region.

The ability of EcoRI restriction enzyme fragmentation patterns and of immunoblotting to differentiate methicillin-resistant isolates of Staphylococcus aureus were compared. All isolates examined were typable by both methods and the reproducibility of each was excellent. Immunoblotting differentiated eight types and DNA restriction patterns four. The former technique was of value in characterising methicillin-resistant isolates of S. aureus and controlling an outbreak due to them.

Seven clinical isolates of chloramphenicol-resistant Haemophilus influenzae were studied. The products of chloramphenicol inactivation by chloramphenicol acetyltransferase (CAT) were identified by high performance liquid chromatography. The sole product in H. influenzae is a single monoacetyl compound, whereas variants of CAT isolated from other chloramphenicol-resistant bacteria usually produce both monoacetyl and diacetyl chloramphenicol metabolites. The chloramphenicol resistance gene was found to reside on a 65-kb plasmid which, in five of the six cases studied, appeared to be integrated into the host cell chromosome.

The synergic relationship between Escherichia coli and Bacteroides fragilis was examined in a model of intra-abdominal abscess formation. The addition of B. fragilis to E. coli in the fibrin clot inoculum increased abscess weight and residual numbers of E. coli in the abscess at 7 days. In a reciprocal fashion, E. coli was capable of enhancing B. fragilis persistence in abscesses. Neither heat-killed E. coli nor heat-killed B. fragilis was able to mimic the synergic effect of its live counterpart. Furthermore, B. fragilis culture filtrate was unable to reproduce the ability of live B. fragilis to act synergically with E. coli. For B. fragilis to act synergically with E. coli, it had to be inoculated locally with E. coli in the peritoneal cavity, indicating that an effect on systemic resistance by B. fragilis was an unlikely mechanism for the production of bacterial synergy. These studies suggest that the synergic relationship between bacteria in polymicrobial infections is a complex one, resulting from intimate interactions between bacteria and the host in the local milieu of the infection.

The susceptibility of 130 clinical isolates of gram-positive cocci to a wide range of antimicrobial agents was assessed by ATP bioluminescence in a 4-h test. ATP assays were performed on a novel luminometer, the Amerlite Analyser, which measures luminescence from microtitration trays. For most organisms tested, there was good correlation (>90%) with conventional MIC values estimated on 18-h cultures. However, a problem was found with detection of penicillin resistance in Staphylococcus aureus by the ATP method, 13% of strains showing major disagreement. Methicillin resistance of S. aureus was shown reliably for most strains (94%) by ATP assay, provided they were incubated at 30°C. The Amerlite Analyser offers the potential for the development of a semi-automated antimicrobial susceptibility test, with a significant reduction in reagent costs when compared with previously described bioluminescence protocols.

The abilities of six strains of Salmonella typhimurium to associate with rabbit ileal have been measured in vitro. Two were “virulent” strains (TML and W118 which are invasive and inducers of fluid secretion in rabbit ileal loops); four were “avirulent” (LT7, M206 and SL1027 which are invasive but induce negligible fluid secretion, and Thax-1 which is neither invasive nor an inducer of fluid secretion). A special organ-culture apparatus was designed to expose only the luminal surface of the mucosa to organisms. Viable counts of washed homogenised tissue taken 30 min after challenge showed that virulent strains TML and W118 and avirulent strains LT7 and M206 could not be distinguished from each other. Avirulent strain SL1027 associated less well than the other four strains, and Thax-1 associated less well than SL1027; both these strains were non-motile whereas the other four were motile. Thus, early association with gut mucosa did not discriminate all avirulent strains from the virulent strains. Qualitative examination of tissues by scanning electronmicroscopy did not detect strains LT7 and M206 on the mucosal surface whereas strains TML and W118 were readily seen, suggesting that the nature of association of virulent and avirulent strains was different. Qualitative examination by transmission electronmicroscopy of tissues challenged in vivo for 120 min showed virulent strains TML and W118 invading epithelial cells; similar events were reproduced after 120-min challenge in vitro. In contrast, invasion by avirulent strains was observed only very rarely.