- Volume 17, Issue 3, 1984

The relative susceptibilities of capsulate and non-capsulate variants of Bacteroides fragilis to serum and phagocytic killing were investigated. The capsule of B. fragilis did not confer resistance to serum killing. Phagocytic killing of non-capsulate B. fragilis occurred at bacterial concentrations of 1 × 106 and 1 × 107 cfu/ml. Capsulate B. fragilis organisms were also phagocytosed and killed at a concentration of 1 × 106 cfu/ml, but phagocytosis and killing were impaired at a concentration of 1 × 107 cfu/ml.

The determinant responsible for the ability of Bacteroides spp. to inhibit polymorph phagocytic killing of aerobic organisms has not yet been identified. Therefore, the roles of lipopolysaccharide and capsular polysaccharide of B. fragilis were investigated. Serum-resistant and serum-sensitive strains of Proteus mirabilis were used to indicate inhibition of phagocytic killing and serum killing of aerobes. Whole organisms of B. fragilis, purified lipopolysaccharide and capsular polysaccharide were added to an in-vitro phagocytosis system. Results showed that > 107 bacteroides/ml inhibited both serum and phagocytic killing. Concentrations below 107/ml had little effect on either process. Purified capsular polysaccharide (10 or 100 μg/ml), either alone in the system or in combination with sub-inhibitory concentrations of B. fragilis also markedly inhibited serum and phagocytic killing. Lipopolysaccharide (9 μg/ml) appeared relatively inert. B. ovatus, reputedly non-capsulated, produced identical results to those obtained with B. fragilis, but an encapsulated strain of Streptococcus pneumoniae did not inhibit serum or phagocytic killing.

A single radial haemolysis (SRH) technique, using Reiter protein or Reiter lysate as the coating antigen, was investigated. Results obtained with syphilitic and presumed non-syphilitic human sera were compared with results obtained in the absorbed fluorescent treponemal antibody test (FTA-ABS), the Reiter protein complement fixation test (RPCFT), the Venereal Diseases Research Laboratory Slide test (VDRL) and the Cardiolipin Wasserman reaction (CWR). The SRH reaction, with either Reiter antigen, was more sensitive than any of the screening tests (RPCFT, VDRL and CWR) for detecting positive syphilitic antibodies. Although the SRH test used almost the same materials as the RPCFT, it was appreciably more sensitive for the detection of the group-specific antibodies in syphilitic human serum.

The mechanisms of resistance to beta-lactam antibiotics in 191 isolates of Pseudomonas aeruginosa were examined. These represented the most resistant organisms of 1866 isolates collected during a national survey of antibiotic resistance in this species. One hundred and seventy-two isolates were selected because they were resistant to carbenicillin (MIC > 128 mg/L) and 19 because the MICs of cefotaxime were greater than the MICs of carbenicillin. Of the carbenicillin-resistant isolates, 35 produced plasmid-mediated beta-lactamases known to be active against carbenicillin and seven produced unusual beta-lactamases; in 131 strains, resistance could not be attributed to beta-lacta-mase production and was considered to be intrinsic. The unusual antibiogram in which the MIC of cefotaxime was greater than the MIC of carbenicillin was associated with overproduction of the chromoso-mally-determined Sabath and Abrahams’ beta-lactamase. Selection of strains with this last mechanism represents a novel resistance problem and one which may increase with increased use of newer antipseudo-monal beta-lactams.

Methicillin-resistant Staphylococcus aureus strains isolated at a single Melbourne Hospital between 1969 and 1981 were examined for susceptibility to a range of antimicrobial agents and for the presence of plasmid DNA. Isolates obtained during 1969 possessed a plasmid of mol. wt 20 × 106, encoding heavy metal resistance and penicillinase production, and a plasmid of mol. wt 2–8 × 106, mediating tetracycline resistance. In the majority of isolates obtained after 1973, these functions were chromosomally encoded. Before 1980, both high- and low-level chromosomally-encoded gentamicin resistances were encountered, whereas isolates from 1980 and 1981 displayed low-level gentamicin resistance only; the latter phenotype was most commonly mediated by a plasmid of mol. wt 18 × 106 that also encoded resistance to tobramycin and kanamycin. Chloramphenicol resistance in strains isolated throughout the period was mediated by one of three plasmids, each of mol. wt c. 3 × 106.

Inspection and blind subculture was carried out on 7031 consecutive blood cultures at 10 p.m. on the day they were received. Analysis of the results after a 2-year period showed that 119 out of 237 (50%) bacteraemic patients were detected at this stage. Preliminary sensitivity tests were done at this time and their results were available within 24 h of the blood cultures being received. Thus earlier specific therapy was possible in 50% of the cases of bacteraemia.

Twenty eight strains of Clostridium difficile, isolated from an outbreak of antibiotic-associated colitis and diarrhoea in an orthopaedic ward and from sporadic cases throughout Sweden, were sent to Edinburgh for immunochemical fingerprinting without information about their origin. EDTA extracts of the organisms were examined by crossed immunoelectrophoresis (CIE), polyacrylamide gel electrophoresis (PAGE) and electroblot transfer. Two patterns were revealed by CIE: group A (18 strains) and group B (10 strains). PAGE and electroblot transfer revealed one major group of 10 strains (group 1), six small groups of two or three strains and six strains which were unlike any other strain. The CIE group B and PAGE-electroblot group 1 were identical. Nine of the 10 strains in this gioup were from patients in the outbreak. These findings indicate that a single strain spread in the orthopaedic ward as a nosocomial infection and that this strain differed from most other strains investigated. The PAGE-electroblot technique should, therefore, greatly aid investigations into the epidemiology of C. difficile infections.

Most phage-Group-II Staphylococcus aureus have been shown to carry at least one plasmid. The proportion of strains that are resistant to tetracycline appears to have increased during the last 17 years. Restriction maps of several of the small plasmids isolated from Group-II strains are presented and compared with those known for staphylococcal plasmids. These small plasmids are similar to previously characterised plasmids from staphylococci of other phage groups.

A total of 464 Haemophilus influenzae strains, most of them fresh clinical isolates, have been classified by chemotyping—a combination of auxotyping and biotyping. Seven auxotests and four other biochemical tests allowed recognition of 56 types. These were to a degree site-specific. H. influenzae of capsular type b proved almost without exception to belong to one chemotype, and 24 of 33 strains assigned to this chemotype were capsulated. When surgical-ward isolates of H. influenzae were typed, the results suggested that some cross-infection had occurred.

Nine strains of group B streptococci type III, five with R-protein (R +) and four without (R —) were tested for capacity to colonise the upper respiratory tract in mice and to adhere to human buccal cells. In the mouse model, 80-μl inocula of dilutions of overnight cultures of the strains in Todd Hewitt broth were placed in the external nares under light ether anaesthesia. A pilot experiment demonstrated that it was reasonable to study the throat colonisation 2 and 4 days after inoculation. Groups of 18-20 mice were then given inocula containing 8 × 106 cfu/ml of five R + and four R — strains. At day 4, significantly more mice were colonised with type III, R + strains (73% of the animals) than with type III, R — strains (44%) (p<0.01). In adherence experiments with human buccal cells, no difference was found between the R+ and R — strains. The results indicated that the higher colonisation rate among R + strains was mediated by mechanisms other than adherence.

None of 34 sera from patients with gonorrhoea contained antibodies bactericidal for strains of Neisseria gonorrhoeae with Group-I lipopolysaccharide (LPS). All contained antibodies against a strain with Group-II LPS, as do sera from uninfected controls. The absence of Group I-LPS antibodies in infected humans contrasts with previous findings that mice immunised with strains from either of the LPS groups produced bactericidal antibody to Group I. Our hope that detection of antibodies to Group-I strains would provide a screening test for gonorrhoea was, therefore, not realised.