A genetic determinant essential for hemolysin production by Listeria monocytogenes has been inactivated by insertion of transposon Tn916 into L. monocytogenes DNA. The transposon was transferred by means of conjugation of a streptomycin-resistant L. monocytogenes recipient strain with Streptococcus faecalis CG110 on membrane filters. Among the tetracycline-resistant transconjugants, mutants were detected which had lost hemolytic activity. When tested in a mouse model, these mutants appeared to have lost the virulence that characterizes the parental strain. An extracellular protein of 58,000 apparent molecular weight was eliminated in the nonhemolytic mutants. In some of the mutants, the decrease in the production of the 58,000-dalton protein was accompanied by the production of a new protein of 49,000 apparent molecular weight. Hemolytic revertants regained the hemolytic phenotype and virulence and produced the extracellular protein that characterizes the recipient strain. Hybridization studies with Tn916 DNA indicated that the transposon is present in EcoRI and HindIII fragments of the nonhemolytic mutants. Single copies of Tn916 were detected in the chromosomal DNA of two of the three nonhemolytic mutants that were studied in detail. In hemolytic, tetracycline-sensitive revertants Tn916 appeared to be completely excised from the chromosome.
Young adult (6–12 weeks old) and aged (20–24 months old) NMRI mice were infected with various intracellular parasites. The following results were obtained: (1) After a sublethal infection with Listeria monocytogenes, aged mice were found to show a resistance similar to that of young adults. A challenge infection with this pathogen was followed by specific immunity of long duration in both age-groups. (2) On the other hand, young animals were significantly more resistant to Salmonellatyphimurium than aged mice. It was concluded that this was due to the LD50 which was 14 times greater for 2-month-old than for 20-month-old mice. Furthermore, during 7 weeks after infection there were more S. typhimurium in the spleens of senescent mice than in those of young adult controls. (3) Aged mice showed highly increased susceptibility to the weakly virulent DX strain of Toxoplasma gondii. Almost all aged animals died whereas the control mice survived. When death of the aged mice was prevented by treatment with sulfadiazine after infection with the DX strain, the aged mice were found to be as well protected against subsequent infection with the strongly virulent BK strain as the young adult mice. These results suggest that the susceptibility of the aged animal to infectious agents may considerably vary from one pathogen to another.
Two 18S rRNA-targeted oligonucleotide probes specific for Candida albicans and Candida parapsilosis were used to detect and identify by fluorescent in situ hybridization these medically important Candida species in deep organs of mice after experimental systemic infection. The C. albicans-specific probe detected fungal cells in kidney, spleen, and brain sections of a mouse infected with C. albicans but not in a mouse infected with the closely related species C. parapsilosis. Conversely, the C. parapsilosis-specific probe detected fungal cells in the deep organs of a mouse infected with C. parapsilosis but not in the deep organs of a C. albicans-infected mouse. In addition, the C. albicans-specific probe was used to detect this species in human blood spiked with yeast cells by a lysis-filtration assay and subsequent fluorescent in situ hybridization. By this assay, as few as three yeast cells per 0.5 ml of blood were consistently detected. Our results demonstrate that fluorescent in situ hybridization with species-specific rRNA-targeted oligonucleotide probes provides a novel, culture-independent method for the sensitive detection and identification of Candida species in clinically relevant material.
The therapeutic activities of various antibiotics were evaluated in two murine models, i.e. the infection of normal mice as well as of nude mice. Coumermycin and rifampicin were the most active drugs, since not only inhibition of multiplication but also rapid elimination of Listeria monocytogenes could be achieved in normal and immunocompromised animals. Ampicillin was the most active beta-lactam antibiotic followed by azlocillin. The other beta-lactam antibiotics were definitely less active. The combination of ampicillin with gentamicin expressed no synergistic effect in vivo. Co-trimoxazole as well as ciprofloxacin were of moderate therapeutic value. The bacteriostatic drugs such as tetracycline and erythromycin were able to inhibit the bacterial multiplication in the normal mouse but not in the immunocompromised host. Thus an optimal drug for therapy of listeriosis does not yet exist.
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