Viruses are the latest weapon against salmonella and campylobacter in poultry

The major food-poisoning bacteria associated with poultry are salmonella and campylobacter species. Between them, these two bacteria cause the vast majority of outbreaks of food poisoning throughout Europe.

However, certain antibiotics which until recently were added to animal feeds as growth promoters, have been withdrawn from use as a result of the increasing consumer concern over the use of chemical additives in food production.

Added to this is the problem of increasing bacterial resistance to antibiotics and, therefore, loss of efficacy in treating animal and human diseases. Three recent comprehensive reports on the problem of bacterial antibiotic resistance commented on the use and abuse of antimicrobials in food animal production, and recommended the use of alternative methods of reducing numbers of food-poisoning causing bacteria in food.

One alternative is phages. They are viruses that only infect bacteria and are the most abundant organisms on earth. They are highly specific affecting only the target bacterium, are non-toxic to animals and plants, and are able to self-reproduce as long as the corresponding host bacteria are present.

Layer chicken

Bacteriological studies have indicated that phages are able to multiply in host bacteria within animals, and new phages to attack resistant host strains are readily selected.

Finally, production of phages to very high numbers, is simple, rapid and relatively inexpensive. Phages have been used successfully as therapeutic agents in Eastern Europe and the former Soviet Union for many years. Their clinical use was not widespread in western countries because of the advent of antibiotics, but the increasing occurrence of bacterial antibiotic resistance has renewed interest in this approach.

This EC project (Phagevet) seeks to isolate, characterise, produce and apply phages highly active against salmonella and campylobacter, to young birds and assess their efficacy in minimising or eliminating these two pathogens.

To date, our Russian partner (NIIgenet, Moscow) has isolated and selected phages against salmonella and the activity spectra have been determined on many strains of these food poisoning bacteria, in particular S enteritidis, the major species associated with poultry.

Similar research at the Universidade do Minho in Portugal has produced several active campylobacter phages, and researchers at the University of Bristol has now tested both these on chicks.

Day-old chicks were infected with S enteritidis and seven days later were orally administered different doses (high, medium and low, to optimise the number of phages) of a salmonella-specific phage. Gut contents were analysed post mortem for both surviving salmonellae and phages.

A similar method was followed for campylobacter, although a different method of administering the phage was used.

rintop chickens

Treatment of the two bacterial infections with phages gave different results. Treatment of the salmonella infection (see Figure 1) with phage resulted in an immediate and large reduction in salmonellae (99.9%) but this effect did not persist for the seven days of the experiment. In contrast, numbers of campylobacter (see Figure 2) were reduced by a smaller amount (99.0%) at Day 2, and the effect persisted for seven days.

Since these food-poisoning bacteria were not entirely eliminated, only reduced, by phage application, there appears to be a balance established between “predator” (phage) and “prey” (host bacteria).

This suggests that application of phages for control of numbers of these two bacteria entering the food chain, might best be made 24-48hours before slaughter, so that the dramatic reductions in salmonella and campylobacter coincide with the birds entering the processing plants.

These encouraging results prove the principle of using phages to control food-poisoning bacteria in live birds, and suggest that a commercial product could soon be available following further development and trials, perhaps within 2-3 years.

• Dr Paul Gibbs is a principle consultant at Leatherhead Food International and also an invited scientist at Universidade Portuguesa in Porto, Portugal.

The problem: Salmonella and campylobacter are the main food-poisoning bacteria associated with poultry. Numbers of these bacteria in live birds can be reduced by using antibiotics, but consumer concerns over residues in food and antibiotic resistance prevents their use. This project sets out to develop an alternative approach.

Who carried it out: Project co-ordinator Joana Azeredo of the Universidade do Minho, Portugal. Partner research groups are: Leatherhead Food International (UK), Universidade Católica Portuguesa (Portugal), University of Bristol (UK), Universidad de Santiago de Compostela (Spain) and State Institute for Genetics and Selection of Industrial Microoganisms(Russia)

Timescale: May 2005 – May 2008

Funder: European Commission (STREP Project No 2005-7224)

Key findings

  • Phages have been used successfully as therapeutic agents in Eastern Europe and the former Soviet Union for many years.
  • Testing found that a salmonella phage caused a rapid fall in the number of salmonellae present in infected chicks, but the effect did not persist.
  • A “cocktail” of campylobacter phages caused a fall in bacterial numbers in infected chicks and the effect persisted.
  • A commercial phage product could be available within 2-3 years.