1998 University of California Poultry Symposium--University of California-Davis

Joan S. Jeffrey, DVM, MS, Diplomate-ACPV

Competitive exlusion (CE) is a term that has been used to describe the protective effect of the natural or native bacterial flora of the intestine for limiting the colonization of some bacterial pathogens of chickens and turkeys. There is a large body of scientific literature on this subject dating back over 25 years. Only in the last few years have these concepts been developed to the point that commercial preparations of CE products are available for use in poultry. Competitive exclusion products are also called probiotics, direct-fed microbials or CE cultures. For simplicity, in this proceeding, the term CE product will be used. Competitive exclusion products may provide a significant tool for the poultry industry in combating the occurence of intestinal disease and reduction of food borne pathogens.

The newly hatched chicken or turkey gut is devoid of bacteria. In the first few hours to days of life, the normal gut bacteria (microflora) that inhabit the intestine become established. This holds true in all animals. Intestinal microflora function to break down ingested food, produce some vitamins and most importantly provide a natural barrier to harmful bacteria that enter the host. In the days when chicks were hatched under a hen, the bacteria shed in the feces of the healthy adult hen provided the inoculum for the establishment of a similar microflora in the chicks. With the advent of modern incubation, the first bacteria the chick or poult is exposed to are those in the incubator, chick box, and litter of the poultry house.

The normal microflora of the intestinal tract is made up of a diverse population of bacteria. Some of these bacteria are anaerobic (grow without oxygen), some are aerobic (oxygen dependent) and some are in between (facultative anaerobes or microaerophilic). All of these bacteria are in competition for survival. They compete for attachment sites and nutrients from the ingesta passing through the intestine. Each species has specific requirements for growth and are affected by relative acidity or alkalinity (pH) of their environment and by products produced by neighboring bacteria. The environment of the intestine can be altered by the compostion of the diet of the bird or by disease. As an example of the effect of diet on gut microflora, researchers have demonstrated that the colonization of some pathogens can be inhibited by the feeding of complex sugars, like mannose and lactose.

In 1973, Nurmi and Rantala demonstrated that treatment of newly hatched chickens with intestinal contents (feces) of adult chickens conferred resistance to infection by Salmonella infantis. Many scientists have since contributed to this area of research. To highlight some of the major findings:

Three mechanisms have been proposed that may contribute to the ability of CE products for preventing the colonization of salmonellae in newly hatched chicks

1. The physical obstruction of attachment sites for Salmonella by the native flora lining the intestine.

2. Competition for essential nutrients by the native flora limits the ability of salmonellae to grow.

3. The protective flora appear to produce volatile fatty acids (especially in the ceca) that limit the growth of salmonellae. The exact mechanism by which CE products infer resistance to pathogens is still unknown and an area of needed research.

The first CE products were simply fecal contents from healthy adult chickens suspended in an aqueous solution and placed in the crop of the newly hatched chicks by gavage. Later, undefined mixtures of intestinal bacteria were serially culture under anaerobic conditions. These preparations were shown to be highly effective. The largest concern about undefined cultures arises from their undefined nature. Serial subculture has been used for the dilution to extinction of harmful parasites or viruses that may be present in the original cultures. The known pathogens of poultry that are shed in the feces are not capable of withstanding the culture techniques for CE products. Screening tests for known pathogens are conducted but the doubt has been raised that all pathogens can be identified by current screening methods.

Other research groups have developed defined mixtures of bacteria for use as CE products. The number of bacterial strains and species is critical to the effectiveness of the product. Products that contain a single or only a few bacteria strains have not been protective. There appears to be difficulty in maintaining a stable culture of numerous strains of bacteria over long periods of time. Some research groups have addressed this issue by applying continuous culture techniques in the production of CE products, however, if this technique limits the number of strains that can be cultured the efficacy of the resulting product will be reduced.

The methods used to culture the componenet bacteria also affect the protective qualities of the product protection they impart. And the composition of the original microflora will vary from bird to bird. Intestinal flora from healthy commericially reared chickens has been superior to that from Specific Pathogen Free chickens. Additionally, there may be batch to batch variation in CE products depending on length of time the bacteria have been grown in culture. Clearly, quality control measures are extremely important for those products being offerred on a commercial basis.

In a 1993 review article, Stavric and D'Aoust compared undefined and defined bacterial preparation for CE of Salmonella in poultry. They concluded that the efficacy of either type of product was much more variable in field situations than in laboratory tests and overall, undefined cultures were more efficacious than defined culture preparations. Products that showed efficacy against salmonellae were not protective against Campylobacter infection.

In laboratory studies, the protective microflora is usually applied directly into the crop of the chick by gavage. For commericial application this technique is not feasible, therefore, CE products have been produced in liquid and lyophilized (dried) forms for practical use. In field studies, CE products have been administered in the drinking water, by spraying hatching eggs or chicks in the hatch boxes, within feed slurries or sprayed on agar plates. All methods have had some success but no method has been effective in 100% of the trials.

Use of CE products in commerical poultry.

Clearly, the use of CE products to protect newly hatched, highly susceptible chicks or poults being placed into commericial production systems could be of great benefit in reducing colonization and disease caused by paratyphoid salmonellae. If this protection translated into a reduced prevalence of food borne pathogens at harvest, this could also be of great value.

The use of CE products to restore a protective microflora following disruption of the intestinal bacteria by a disease, such as viral enteritis or coccidiosis may also become a helpful husbandry practice. Alterations of intestinal flora due to stress in poultry flocks from transport, handling (vaccination), molting, chilling or heat stress may also be treated by CE products.

The effect of feed additives such as antibiotics and coccidiostats regularly fed in commercial poultry diets and the effect of antibiotic used for disease treatment on the protection inferred by CE products must also be considered because antibiotics can alter intestinal flora. The presence of high levels of chlorine or other disinfectants in water available to poultry flocks may also warrant investigation. Stavric and D'Aoust reported that medicated feed containing about 200 ppm of bacitracin, furazolidone, gallimycin, penicillin/streptomycin, cholortetracycline and tylosin, or 10 ppm of nitrivin didn not adversely affect the efficacy of CE treament. Common antibiotic feed additives used as growth promotants (5-50 ppm) had mixed effects on CE treatment. For example, bactracin or virginiamycin improved the performance of CE cultures, flavomycin had no effect and avoparcin reduced the level of protection.

The positive benefits from commercial preparations of CE cultures are well accepted in Europe after more than 10 years of use. CE products have been efficacious against colonization by numerous salmonellae serovars, including PT4, but not against campylobacter.

For U. S. poultry producers, competitive exclusion provides an opportunity for protecting poultry against pathogenic bacterial colonization, particularly salmonellae. Because the microflora of the intestine are in a dynamic state, undergoing continuous change, and subject to alteration by numerous forces, the use of CE products is not a panacea against salmonellae infection throughout the life of the bird. However, in conjunction with other management practices that limit the introduction and exposure of poultry flocks to salmonellae, CE products can make a valuable contribution to flock health and the safety of poultry products as food. Continued research in this area to identify the specific mechanisms by which CE products infer protection and to improve methods of delivering the product in commercial situations will add to the value of these products.

Return to VMTRC Poultry Home Page

Return to VMTRC Home Page