- Volume 42, Issue 4, 1995

Administration of either gentamicin or amikacin induced an increase in the number of amikacin-resistant (AR) isolates of certain Enterobacteriaceae and Acinetobacter species in a hospital in Buenos Aires. A total of 127 AR isolates was selected to study the molecular mechanisms of resistance involved. The aac(6′)-Ic gene was found by dot-blot hybridisation in every Serratia marcescens isolate. A gene different from aac(6′)-Ia,aac(6′)-Ib and aac(6′)-Ic encoding the AAC(6′)-I activity was found in a 15-5-kb plasmid in Acinetobacter spp. Plasmids from 27 Enterobacteriaceae contained an aac(6′)-Ib gene and 26 of these carried sequences related to the Tn1331 transposon, whereas one Escherichia coli plasmid showed homology in another fragment of the Tn1331 transposase. Because plasmids bearing the aac(6′)-Ib gene were heterogeneous, dissemination of the aac(6′)-Ib gene may have been due to transposition of Tn1331 rather than the spread of an epidemic plasmid. The rate of AR isolates varied within each species in spite of the presence of Tn1331, and it is likely, therefore, that this transposon may not be the sole factor responsible for the observed variation. The aph(3′)-VIa gene (originally described in Acinetobacter spp.) was found with high frequency (80%) in this Acinetobacter population. Furthermore, this gene was found also in plasmids from 20% of other gram-negative organisms commonly involved in nosocomial infections in this hospital.

DNA hybridisation of 309 consecutive Staphylococcus aureus clinical isolates with oligonucleotide probes specific for genes encoding Panton-Valentine leucocidin (luk-PV) and y-haemolysin (hlg) revealed that 99% of randomly selected strains carried the hlg locus whereas only 2% harboured the luk-PV as well as the hlg loci. Only 1% of the strains did not possess either gene. In a clinical prospective study of independent S. aureus strains, 58 Panton-Valentine leucocidin (PVL)-producing isolates were shown to be responsible for primary skin infections, mainly furuncles (86 %). Phage susceptibility patterns and pulsed field gel electrophoresis (PFGE) profiles of DNA were shown to be polymorphic epidemiological markers of PVL-producing strains. In eight patients with recurrent furuncles, the PVL-producing strains isolated either from furuncles or from the anterior nares were considered to be identical in each based upon phage sensitivity profiles or PFGE patterns.

Vibrio cholerae O139 organisms isolated from different parts of India and from Bangladesh were characterised with respect to their haemagglutination (HA) activity, plasmid content, cholera toxin (CT) production, cell surface protein and lipopolysaccharide (LPS) profiles, and antigenic properties. Of 28 V. cholerae O139 isolates tested, 14 (50%) were shown to agglutinate chicken erythrocytes; the HA activity was sensitive to D-mannose 0.1 %. In parallel experiments, 12 (92.3%) of 13 V. cholerae O1 (El Tor) and 12 (75%) of 16 non-O1, non-O139 strains agglutinated chicken erythrocytes. Plasmid analysis of 32 O139 isolates showed that 12 (37.5%) carried one or more plasmids of 35.8-2.6 MDa. Plasmids were not detected in any of the V. cholerae O1 strains, although plasmids were demonstrable in 35% of the non-O1, non-O139 strains tested. V. cholerae O139 isolates showed an ability to produce CT that depended on media composition and other cultural conditions. A comparison of envelope and outer-membrane protein profiles between O1 and O139 isolates failed to show any significant differences. LPS analysis of O139 isolates revealed that these organisms were devoid of long “O” side-chain polysaccharides. Some of the non-O1, non-O139 strains also showed similar LPS profiles whereas others showed the presence of long repetitive “O” side-chain polysaccharides similar to those seen in O1 organisms. An antiserum raised against V. cholerae O1 strain O395 did not show any significant reactivity towards O139 and non-O1, non-O139 strains although it reacted with other O1 strains. Furthermore, the anti-O1 serum induced marked protection against challenge with an O1 strain but not with an O139 strain in passive protection experiments. All these results indicate that, despite sharing some common properties with V. cholerae O1, the O139 isolates posses certain characteristics that make them distinct from their O1 counterparts.

Insertion mutagenesis with the help of the plasmid pFS23 was used to generate Yersinia pestis fra mutants. The results of pFra-strain production under non-selective conditions suggested that such Y. pestis variants may be generated in natural plague foci at high frequency and may participate in supporting the epizootic process. Present data suggest that the reduction of virulence in Fra-strains reported by the majority of investigators was connected with the use of Y. pestis variants carrying additional unidentified mutations. It was shown that the loss of the ability to produce capsular antigen (FI) alone or in combination with absence of murine toxin production did not lead to an increase in LD50 absolute values. Simultaneous loss of these two virulence determinants did not influence the duration of survival of the infected animals. However, absence of only FI antigen production in the infecting strain resulted in prolonged survival of the infected animals. Conversion of plague infection from acute to chronic form is probably dependent on the animal host species and the Y. pestis parent strain subjected to mutagenesis.

The incidence of infections with Candida albicans and also with non-albicans yeast species is increasing rapidly, particularly in immunocompromised patients. Eight Candida and Torulopsis species were compared for their ability to stimulate production of complement components C3 and factor B by monocytes. In addition, the release of granulocytemacrophage colony-stimulating factor (GM-CSF) was determined, because this cytokine affects monocyte complement production. The highest ranked pathogenic yeasts, i.e., C. albicans, C. tropicalis and C. parapsilosis, were the most effective inducers of C3, factor B and GM-CSF production. C. krusei and T. glabrata showed intermediate activity, whereas C. kefyr, C. guilliermondii and T. Candida had only a moderate stimulatory effect on C3 production and did not affect either factor B or GM-CSF release. The stimulated cytokine and complement production in response to the yeasts was highly variable in monocytes from different donors, but there was a consistent inverse relationship between C3 and GM-CSF concentrations in the monocyte supernates. This is in agreement with the previously described suppressive effect of GM-CSF on yeast-induced C3, but not factor B production. The monocyte responses elicited by a specific yeast species may be linked to its pathogenicity, and may also explain the predilection of some yeasts for particular underlying diseases.