- Volume 49, Issue 11, 2000

Acinetobacter baumannii is an opportunist nosocomial pathogen of world-wide importance and produces severe infections in immunocompromised patients. However, the virulence factors contributing to its pathogenic properties are not well known. The effect of normal human serum against 18 clinical isolates of the most prevalent biotypes of A. baumannii in Chile was investigated. The effect of pre-treatment of the cells with ethylene diamine tetraacetic acid (EDTA) or bismuth subsalicylate (BSS), compounds known to decrease the amount of lipopolysaccharide (LPS) and bacterial capsular polysaccharide (CPS), respectively, in other gram-negative bacteria, was evaluated. Most isolates (16 of 18) showed resistance to normal human serum. Prior treatment with EDTA rendered nine of these isolates susceptible to serum, while seven isolates maintained their resistance. Pre-treatment with BSS did not modify the serum-resistant behaviour of the isolates. The results suggest that LPS might be involved in the resistance of A. baumannii to human serum whereas CPS does not seem to contribute to this property.

The role of bacterial capsule in inflammatory responses in experimentally induced pneumonia caused by Klebsiella pneumoniae was evaluated by comparing the host immunological responses in mice infected with capsulate strain DT-S and non-capsulate mutant strain DT-X. Anaesthetised ICR mice were infected intranasally with inocula of strain DT-S or DT-X. Mice infected with strain DT-X survived significantly longer than those inoculated with strain DT-S. Viable bacterial counts in lungs and blood increased rapidly in mice infected with strain DT-S, in contrast to the gradual decrease in their density in lungs and intermittent bacteraemia in mice infected with strain DT-X. The number of broncho-alveolar lavage (BAL) cells in mice infected with strain DT-X at 24 h after inoculation was significantly higher than in those infected with strain DT-S. In the early stages of infection, the levels of tumour necrosis factor-α and interleukin-6 in BAL fluid of mice infected with strain DT-X were significantly higher than those of mice infected with strain DT-S. In contrast, in the late stage of infection, the levels of these cytokines in serum of mice infected with strain DT-S were significantly higher than in mice infected with strain DT-X. These results suggest that K. pneumoniae capsule may suppress the host immunological responses, thus allowing the bacteria to grow, causing pneumonia, septicaemia and death.

An organ culture system involving explants of distal rabbit ileum was used to study the roles of lipopolysaccharide (LPS) and plasmids in primary invasiveness for enterocytes in situ of strains of Salmonella serotypes Typhimurium and Enteritidis. Long-chain LPS per se does not confer invasiveness on Typhimurium, as known avirulent, hypo-invasive strains express smooth LPS. However, the invasiveness of a naturally occurring rough isogenic derivative of Salmonella serotype Enteritidis PT 4 was about half that of its wild-type parent. Therefore, smooth LPS appears to play a secondary role in maximising invasiveness. No evidence was found to correlate primary invasiveness for gut of 18 strains of Typhimurium with plasmid profiles in general or with the 60-MDa serovar-specific virulence plasmid in particular. Evidence is presented that strongly suggests a seasonal variability in susceptibility of rabbit gut to invasion by Typhimurium. Although no explanation is given for this summer insusceptibility, the data indicate the importance of the physiological status of the host in relation to susceptibility to invasion by Salmonella.

It has been postulated that certain organic acids produced by the anaerobes associated with bacterial vaginosis (BV) could prevent a purulent response in this infection. Varying concentrations of pure succinic, acetic and lactic acids were incubated in vitro with a monocytic cell line (MonoMac 6). High inhibition of chemotaxis was produced by succinic acid; lower inhibition and no inhibition was shown by acetic acid and lactic acid respectively. Succinic and acetic acids were detected in high concentrations in the vaginal fluid of women with BV and in culture supernates of Prevotella and Mobiluncus spp.; these acids impaired chemotaxis of MonoMac 6 cells in vitro. The vaginal fluids of normal women and the culture supernates of Lactobacillus spp. had no effect on chemotaxis.

Two isolates of Candida glabrata from the same stool sample from a bone marrow transplant recipient treated with fluconazole, and designated 1084-L for large colonies on yeast extract-peptone-dextrose-agar and 1084-S for small colonies, were analysed. In-vitro susceptibility tests with a commercially available disk diffusion procedure showed that isolate 1084-L had a susceptibility pattern typical of wild-type strains of C. glabrata with sensitivity to polyenes and the presence of resistant colonies randomly distributed within the inhibition zones for all azole compounds except tioconazole. In contrast, isolate 1084-S, which was found by pulsed-field gel electrophoresis and random amplification of polymorphic DNA to be genetically closely related to isolate 1084-L, exhibited cross-resistance to the azole compounds except tioconazole. Determination of MICs by the E-test method confirmed these results, showing that isolate 1084-S had greater sensitivity to amphotericin B and complete resistance to ketoconazole and fluconazole. Growth on agar plates containing glucose or glycerol as the sole carbon source suggested that the resistant isolate had a respiratory deficiency, which was further demonstrated by flow cytometric analysis of the fluorescence of rhodamine 123-stained blastoconidia. Restriction endonuclease analysis of mitochondrial DNA (mtDNA) established the mitochondrial origin of the respiratory deficiency. However, PCR amplification of the mtDNA with primers ML1 and ML6, as well as transmission electron microscopy, suggested a partial deletion of the mtDNA analogous to that described for rho − petite mutants of Saccharomyces cerevisiae. Together, these results provided evidence that the selection of azole-resistant petite mutants of C. glabrata may occur in vivo after fluconazole administration, which might explain, therefore, clinical failure of antifungal therapy.

Opportunist infections involving Candida albicans often develop in HIV-positive patients and oral lesions tend to become more frequent as the disease progresses. Previous studies have shown contrasting results concerning the variability of the pulsed-field gel electrophoresis (PFGE) subtypes of C. albicans observed in HIV-positive patients. Carriage of C. albicans was determined by an oral rinse technique; 41 strains of C. albicans (78% serotype A and 22% serotype B) were isolated. There was a direct correlation between candidal load (cfu/ml) and the blood HIV load, whereas there was an inverse correlation with the stage of disease and the CD4 cell counts. The PFGE patterns of isolates were variable with regard to the number and positions of bands. The variability of the band sizes in some run positions showed a Gaussian distribution. Generally, the most frequent size variants were associated with the strains with the highest cfu/ml and lowest CD4 counts (≤200 cells/μl). These findings suggest a possible strain selection over time during disease progression, especially in HIV-positive subjects with low CD4 counts.