E. John Threlfall

The emergence and spread of antibiotic resistance in food-borne bacteria.

Since the early 1990s there has been a dramatic increase in resistance to antimicrobial drugs in Salmonella enterica and Campylobacter spp., and to a lesser extent in Vero cytotoxin-producing Escherichia coli O157 from cases of human infection in developed countries. For S. Typhimurium a particularly important aspect of this increase has been the widespread dissemination of a multiply drug-resistant (MR) strain of definitive phage type (DT) 104 in food animals since the early 1990s. The use of antimicrobials for prophylaxis in food producing animals has been an important factor in the emergence of strains with resistance to certain antimicrobials. It is hoped that recently introduced Codes of Practice for the prophylactic use of antimicrobials in food animals will result in a decline in the occurrence of drug resistant strains in the food chain.

Invasiveness of Salmonella serotypes Typhimurium and Enteritidis of human gastro-enteritic origin for rabbit ileum: role of LPS, plasmids and host factors.

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.

Application of Molecular Methods to the Study of Infections Caused by Salmonella spp.

Disease caused by any member of the genus Salmonella is termed salmonellosis. The type of disease and its symptoms are generally related to the mfecting species and reflect the invasiveness and virulence of the organism. For example, enteric fevers are systemic diseases usually resulting from infection with Salmonella typhi, S paratyphi A, B, or C. Salmonellosis is caused by more than 2200 different salmonella serotypes, which can be classified into three groups according to their adaptation to human and animal hosts. One group of serotypes can be regarded as those as organisms that cause enteric fever only in humans and higher primates. Members of this group, which includes S. typhi, S paratyphi A, B, and C are restricted to humans and higher primates and are not found in food animals. A second group causes diseases in specific animals (e.g., S. dublin-cattle, S. pullorum--poultry, S choleraesuis-pigs). However, when some members of this group cause infections in humans the disease is frequently invasive and can be life-threatening (e.g., S. cholerae-suls, S dublin). The third group, which includes the great majority of the remaining 2000+ serotypes, typically causes mild-to-moderate enteritis in humans, which is often self-limiting, but which can be severe in the young, the elderly, and in patients with other underlying complications This group includes the four serotypes most common in humans in England and Wales at the present time: S. enteritidis, S, typhimurium, S. virchow, and S. hadar. The great majority of serotypes of this third group are zoonotic in origin and have as their reservoirs animals used for food, particularly cattle, poultry, and pigs.

The Problem of Multiresistant Salmonella typhi — use of Molecular Markers in Epidemiology

Typhoid fever is endemic in many developing countries, particularly in the Indian sub-continent, South and Central America and Africa. In contrast only 200 – 300 cases occur in the UK each year and the majority of infections are in patients who have returned from countries where Salmonella typhi is endemic (Anonymous, 1985). Treatment with an appropriate antibiotic is essential and should commence as soon as the clinical diagnosis is made. Of necessity this may be before the results of laboratory sensitivity are available.

Changing Trends in Antibiotic Resistance in Salmonella Isolated from Humans in England and Wales

The Division of Enteric Pathogens (DEP) of the Public Health Laboratory Service (PHLS) is the national reference centre for salmonellas isolated from humans in England and Wales. The DEP also phage types strains of Salmonella typhimurium, S enteritidis and S virchow from food animals and referred by laboratories of the Veterinary Investigation Service. The majority of isolates from both humans and food animals are screened for resistance to a range of commonly-used antimicrobial drugs including ampicillin (A), chloramphenicol (C), gentamicin (G), kanamycin (K), streptomycin (S), sulphonamides (Su), tetracyclines (Te), trimethoprim (Tm), furazolidone (Fu) and nalidixic acid (Nx). This provides a framework for observing trends in the occurence of resistance and for investigating factors which have contributed to the acquisition of resistance.