Molecular analysis of ampicillin-resistant sporadic Salmonella typhi and Salmonella paratyphi B clinical isolates

Objective: To study the mechanisms of antibiotic resistance in Salmonella typhi and Salmonella paratyphi B clinical isolates, and the clonality of resistant strains.
Method: Antibiotic susceptibility was tested by disk-agar diffusion. Conjugation experiments and plasmid analysis by agarose gel electrophoresis after Eco RI digestion were followed by hybridization to a digoxigenin-labeled TEM-type β-lactamase probe. DNA fingerprints were obtained by pulsed-field gel electrophoresis of Xba I-digested chromosomal DNA.
Results: Three S. typhi isolates (7% of the isolates studied), of which one was ampicillin resistant and the other two multiresistant (ampicillin, chloramphenicol, tetracycline, sulfamethoxazole/trimethoprim and streptomycin), and two ampicillin-resistant S. paratyphi B isolates (25% of the isolates studied) were further evaluated. A 34-MDa conjugative plasmid, previously isolated from Salmonella enteritidis , conferred ampicillin resistance. A 100-MDa conjugative plasmid encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim, as well as ampicillin. Chromosomal fingerprinting revealed two distinct resistant strains for each serovar which were different from a matched set of sensitive S. typhi strains.
Conclusions: Two conjugative, TEM-type β-lactamase-encoding plasmids conferred ampicillin resistance to S. typhi and S. paratyphi B. The 34-MDa plasmid was identical to that previously characterized from S. enteritidis , while the 100-MDa plasmid also encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim. Resistant isolates did not belong to a single clone but rather represented distinct strains.     

Host restriction phenotypes of Salmonella typhi and Salmonella gallinarum.

Salmonella typhi and Salmonella gallinarum phenotypes correlated with mouse host restriction have been identified by using in vitro and in vivo systems. S. typhi is capable of entering the murine intestinal epithelium via M cells, as is Salmonella typhimurium, which causes systemic infection in the mouse. But, unlike S. typhimurium, S. typhi does not destroy the epithelium and is cleared from the Peyer's patches soon after M-cell entry. S. gallinarum appears to be incapable of entering the murine Peyer's patch epithelium. Our in vitro evidence suggests that S. gallinarum is taken up in murine phagocytic cells by a mechanism different from that of S. typhimurium. S. typhimurium is taken up at a higher frequency and is maintained at higher viable counts throughout a 24-h time course in a murine macrophage-like cell line than are S. gallinarum and S. typhi.     

Characterization of human antibodies to Salmonella typhi by gel-filtration and antigenic analysis

A normal adult human male was immunized with a heat-killed typhoid vaccine. The appearance of antibodies to `H'' and `O'' antigens was subsequently studied. Antibodies were identified as belonging to specific immunoglobulin classes by gel-filtration on G-200 `Sephadex'', followed by specific immunochemical identification using a two-stage agglutination test. Serum antibodies to `H'' antigen were initially of γ_1M class; later the 7S γ class became predominant. γ_1A antibodies were transient in appearance. Serum antibodies to `O'' antigen were of γ<sub>1M</sub> and γ<sub>1A</sub> classes. Urinary antibodies to `H'' antigen were of γ_1A and 7S γ classes. No antibody activity could be detected in the low molecular weight γ-globulins of urine.     

A new selective medium for the isolation of Salmonellae other than Salmonella typhi.

A medium consisting of a MacConkey's base with added magnesium, calcium, cobalt, zinc, thiosulphate, novobiocin, iron dextran, Tween 80 and EDTA has been found highly selective for Salmonellaparatyphi B and most of the food poisoning salmonellae that commonly occur in Britain. Nearly all the normal faecal flora is inhibited.     

Significance of Salmonella typhi bacteriuria.

Bacteriuria due to Salmonella typhi usually occurs following recent typhoid fever or in chronic carrier states. Data from 18 patients with S. typhi bacteriuria, seen during 5 years, were analyzed. Fourteen patients had localized urinary tract infection due to S. typhi. Four others had bacteriuria, probably associated with typhoid fever. Localized abnormalities of the urinary tract and kidneys and also systemic diseases were found to predispose patients to S. typhi bacteriuria. Local abnormalities encountered included urolithiasis (n = 3), prostatic hypertrophy (n = 1), and tuberculosis (n = 1). One renal transplant recipient and another with lupus nephritis had S. typhi bacteriuria. One had associated strongyloidosis, and another was pregnant.     

Successive Salmonella give and Salmonella typhi infections, laboratory-acquired.

A case of laboratory-acquired typhoid fever is described. The case was complicated by a self-limiting Salmonella give gastroenteritis which may also have been laboratory-acquired and which occurred during the incubation period of the Salmonella typhi infection. The symptoms of typhoid were not sufficiently severe for the patient to seek medical attention and she was recovering from the infection when the typhoid bacillus was isolated from her stools. The mode of transmission of the S. typhi was presumed to be a laboratory infection from an unknown source. Although there was no obvious breakdown in safe laboratory techniques, the infecting dose of S. typhi is known to be small and the dangers of handling specimens which may contain this bacterium are emphasized.     

The serodiagnosis of infection with Salmonella typhi

BACKGROUND/AIMS: The serodiagnosis of infection with Salmonella typhi, using the Widal agglutination assay, relies on patients' antibodies to the O = 9,12 lipopolysaccharide (LPS) antigens, H = d flagellar antigens, and the Vi capsular antigens. A Vi agglutination titre of > 1/40 has traditionally been regarded as indicative of recent infection with S typhi. In this study, 91 sera were used to assess the reliability of the Widal agglutination assay based on antibodies to the Vi antigens. METHODS: The Widal agglutination assay was carried out using protocols established by the Central Public Health Laboratory, Colindale. Antibodies to the Vi capsular antigen were detected using a standard preparation of S typhi, ViI Bhatnagar variant strain (S typhi, ViI). Sera used in the study comprised 73 from patients who were culture positive for S typhi, 10 from patients who were culture positive for other species of Salmonella not expressing a Vi antigen (namely, S javiana, S enteritidis, S typhimurium, S stanley, S saint paul, S bareilly, or S mbandaka), and eight from healthy blood donors. RESULTS: Agglutination titres of > or = 1/40 were detected to S typhi ViI in 69 of 73 sera from patients with typhoid, although 27 of these also agglutinated an unrelated control antigen. The Widal assay also detected significant amounts of agglutinating antibodies to S. typhi ViI in all eight control sera and seven sera from patients infected with S bareilly, S enteritidis, S javiana, S mbandaka, S saint paul, and S stanley. CONCLUSIONS: Agglutinating antibodies to the Vi antigen can be detected by the Widal assay, but even with the appropriate control antigens the results were unreliable. The serodiagnosis of infections with S typhi should be based on the detection of antibodies to both the O = 9,12 LPS antigen and the H = d flagellar antigen by immunoblotting, and should not use the Vi antigen-based Widal assay. Conclusions should be made in the light of patients' clinical details and any knowledge of previous immunisation for typhoid.     

Spurious sulfamethoxazole-trimethoprim resistance of Salmonella typhi.

Several studies have identified thymidine excess in susceptibility test media as the cause of spurious resistance of various bacteria to sulfamethoxazole-trimethoprim. We document the phenomenon in Salmonella typhi and Salmonella paratyphi-A and demonstrate its occurrence in 3 of 17 (18%) lots of Mueller-Hinton agars now in use in major medical laboratories in Lima, Peru. The findings are particularly significant because sulfamethoxazole-trimethoprim is an important alternative to chloramphenicol or ampicillin for the treatment of typhoid and paratyphoid fevers.