- Volume 30, Issue 4, 1989

The production of virulence factors was determined quantitatively for clinical isolates of methicillin-resistant (MRSA) and methicillin-sensitive (MSSA) strains of Staphylococcus aureus from The London Hospital. The examined factors were: production of enterotoxins A, B, C and D, determined by ELISA; quantitation and differentiation of the membrane-damaging α, β, γ and δ haemolysins; and coagulase production determined by a chromogenic assay. Enterotoxin A was produced by MRSA but not by MSSA. All the strains produced haemolysins α, γ and δ at similar levels, but MRSA produced significantly more coagulase than MSSA. MRSA and MSSA were compared in a phagocytosis assay but there was no difference between the phagocytosis of MRSA and MSSA by human polymorphonuclear leucocytes. These findings indicate that MRSA from The London Hospital is at least as well equipped to cause disease as other isolates of S. aureus, and probably better equipped than most hospital isolates of MSSA.

Staphylococcus saprophyticus, a coagulase-negative staphylococcus (CNS), causes acute urinary tract infection predominantly in young women (15-30 years). In the clinical microbiology laboratory identification and differentiation of S. saprophyticus from other CNS usually depends solely upon the demonstration of resistance to the antimicrobial agent novobiocin. Phenotypic characteristics of 36 novobiocin-resistant CNS isolated from the urine of patients with acute urinary tract infections were further analysed and the homogeneity of the isolates assessed. The organisms were speciated by the API STAPH identification system. Twenty-one isolates were S. saprophyticus (p > 97%), and there was one strain each of S. epidermidis, S. hominis and S. simulans (p > 97%). Of the remainder, three isolates were unidentifiable and a further nine had the characteristics associated with more than one species of CNS. Additional tests, including carbohydrate fermentation, antibiotic sensitivity and fluorogenic substrate utilisation, were performed on all isolates. Computer analysis of the results confirmed that testing for resistance to novobiocin selects a heterogeneous group of CNS composed of several different species.

Penicillin-tolerant and -sensitive strains of oral streptococci were treated with penicillin to determine the production of a post-antibiotic effect (PAE). No PAE was seen with any of the S. sanguis strains tested but most strains of the other oral streptococcal species produced a PAE. Cultures on nitrocellulose filters treated with penicillin were examined by scanning electronmicroscopy and showed that tolerant and sensitive strains lost the ability to adhere to the filter after application of antibiotic. When the filters were treated with β-lactamase, before processing for microscopy, the tolerant strains but not the sensitive ones recovered and grew in a confluent lawn similar to the control cultures that had not received antibiotic. Transmission electronmicroscopic examination of similarly treated cultures produced comparable results. Bizarre morphological changes were a feature of the tolerant strains of S. sanguis.

Strains of Streptococcus pyogenes from an outbreak of infection on a burns unit(15), a collection of routine isolates from another hospital(12) and isolates from a national survey of throat infections in children in the community(4) were examined blind by pyrolysis-mass spectrometry (Py-MS). The outbreak strains (M22 T12) previously found to give identical typing results in conventional tests, formed a closely similar cluster and were distinct from other hospital and community strains. One hospital and one community strain were loosely associated with this cluster. Another cluster comprised six antibiotic-susceptible strains and two community strains. Six strains did not fall within the clusters; four were antibiotic-resistant strains isolated in hospital, one an antibiotic-resistant strain isolated in the community, and one a susceptible hospital strain. Results show that Py-MS is a potentially valuable method for rapid comparison of strains in studies of infection.

The haemagglutinins and fimbriae produced by 18 strains of Proteus penneri were studied and compared with those formed by representative strains of other species of Proteeae. After repeated subcultures at 30°C, 12 P. penneri strains formed only MR/K haemagglutinins which were associated with thin, non-channelled, type-3 fimbriae. Two strains formed simultaneously both MS and MR/K haemagglutinins associated with thick, channelled, type-1 fimbriae and type-3 fimbriae, respectively. Four strains formed simultaneously both MR/K and MR/P haemagglutinins. No P. penneri strain formed either MS or MR/P haemagglutinins alone under these conditions. The type-3 fimbriae from P. penneri strain E180 were isolated, purified and found to be protein of 19 Kda. Immunoelectronmicroscopy studies with antibody to the type-3 fimbriae of strain E180 showed that P. penneri strains formed at least two antigenic types of type-3 fimbriae. The type-3 fimbrial antigen of the vaccine strain E180 was shared by another eight strains of P. penneri. A further eight P. penneri strains and the strains representative of other genera within Proteeae had type-3 fimbriae of a different antigenic type. The formation of these haemagglutinins and fimbriae suggests that this organism is well endowed to be a urinary-tract pathogen.

The tracheobronchial secretions from patients with cystic fibrosis often contain high amounts of free proteases. To evaluate whether human leucocyte elastase (HLE) can favour the persistence of bacterial airways infection, we exposed the frog palate mucosa to HLE and then to radiolabelled Pseudomonas aeruginosa and followed the sequence of events by scanning electronmicroscopy. In response to HLE there was a marked outpouring of mucus and a desquamation of the epithelium. P. aeruginosa was shown to adhere to recently secreted granules of mucus and to the exposed submucosal underlying connective tissues. For the eight different bacterial strains studied, a significative adherence to HLE-injured mucosa was observed only in strains that possessed internal haemagglutinating activity. Neither the presence of fimbriae, nor of the mucoid exopolysaccharide, nor of the bacterial surface haemagglutinating activity could be related to adherence of P. aeruginosa to the injured mucosa. These results support the hypothesis that HLE enhances bacterial infection of the respiratory mucosa both by inducing mucus hypersecretion and by exposing receptors to the microbial adhesins. It is also suggested that P. aeruginosa internal lectins may be implicated in adherence to host tissues.

The distribution of K1, K5 and O antigens was examined in 500 clinical strains of Escherichia coli. Of 400 strains from urine, 52% belonged to serogroups O1 O2, O4, O6, O7, O8, O9, O18, O25 and O75; 34% were non-typable (NT) and 14% were autoagglutinable (AA). Antigen K1 was carried by 17% of these strains, and K5 by 15%. The numbers of O-serogroupable, NT and AA strains among 100 strains from blood were 62, 29 and 9, respectively. K1 antigen was detected on 20% of isolates from blood and K5 on 13%. There was no statistically significant difference in the distribution of K1, K5 or O antigens between strains from blood compared with those from urine. The occurrence of K1 and K5 antigens among smooth and AA strains suggested that AA strains were derived primarily from the common O-serogroups.