Because of their pH-reducing and antimicrobial effects, gut acidifiers appear to offer promise as substitutes for antimicrobial growth promoters where the use of the latter is banned or severely restricted. In a recent case study, the use of a gut acidifier improved eggshell quality in a broiler breeder flock.

Antibiotic growth promoters (AGP) have been used for the past five decades to improve the performance of the poultry. Sub-therapeutic levels of antibiotics in poultry feed have increased feed efficiency and growth (table 1).

Table 1:

Effects of growth-promoting antibiotics

The AGPs have been under scrutiny for many years and have been removed from the market by the regulatory authorities in many countries. The usefulness of AGPs has seldom been contested but they are similar to antibiotics used in human medicine and the possibility has been raised that they may contribute to the pool of antibiotic-resistant bacteria.

As a result, the industry has been actively looking for efficacious alternatives to AGPs. Numerous products are considered and among these, the organic acids appear to offer a promising alternative to antibiotics.

What is gut health?

Health of the gut is one of the major factors governing the performance of birds and thus, the economics of poultry production. The profile of intestinal microflora plays an important role in gut health. The gut microflora comprises both commensals (Gram-positive) and entheropathogens (Gram-negative). In healthy birds, there is a balance between the Gram-positive and Gram-negative populations of microflora at an ideal pH. A healthy gut has a predominance of Gram-positive bacteria. The balance gets disturbed when there is a change in the pH due to ingestion of toxic chemicals or chemotherapeutic agents or a change in feed composition. A disease condition results when there is a shift is towards the enteropathogenic population. The multiplication of harmful bacteria may start from the crop itself: food stays in the crop for a longer time and the presence of moisture, body temperature and time to multiply favours the multiplication of these microflora. Thus, maintenance of the ideal pH for microbial balance is essential for keeping the gut healthy (see tables 2 and 3).

Table 2:

pH and residence time of feed in gastrointestinal tract

Table 3:

Optimum pH for bacterial growth

The role of gut acidifiers

The use of gut acidifiers has been proven to be of immense help in maintaining the microbial balance of the gut.

Acidifiers are acids that are included in the feed in order to lower the pH of the feed, gut and microbial cytoplasm by inhibiting the growth of pathogenic intestinal bacteria. This inhibition reduces the microflora competing for the host nutrients and results in better growth and performance of the chicken.

Most gut acidifiers consist of the organic acids (table 4). In descending order of bactericidal effect, some commonly used compounds are benzoic acid, fumaric acid, lactic acid, butyric acid, formic acid and propionic acid. Benzoic acid is superior to other acids beacause it has an effect on coliforms as well as lactic acid bacteria in both the stomach and small intestine. Combining the acids may increase the range of desirable antimicrobial effects.

Table 4:

Anti-bacterial spectrum of organic acids

Functions of organic acids

 -maintain an optimum pH in the stomach, allowing correct activation and function of proteolytic enzymes

-optimise protein digestion in stomach

-stimulate feed consumption by improving palatability of feed

-inhibit the growth of pathogenic bacteria, yeasts and moulds

-improve protein and energy digestibilities by reducing microbial competition with the host for nutrients, as well as endogenous nitrogen losses

-lower the incidence of sub clinical infections

-reduce the production of ammonia and other growth-depressing microbial metabolites

-increase pancreatic secretion and tropic effects on gastrointestinal mucosa

-favour mineral absorption by creating an ideal pH in the intestine (table 5).

Table 5:

Effects of organic acids on poultry performance

Mode of action

The antibacterial action of organic acids depends on whether the bacteria are pH-sensitive or not. Certain bacteria are sensitive to pH, e.g. E. coli, Salmonella spp., Listeria monocytogenes, Clostridium perfringens, while others are not, e.g. Bifidobacteria, Lactobacillus spp. pH-sensitive bacteria.

Organic acids in undissociated state, i.e. non-ionised, more lipophilic, penetrate the semi-permeable membrane of the bacterial cell wall and enter the cytoplasm. At the internal pH of bacteria (around pH 7,0), the organic acids dissociate, releasing hydrogen ions (H⁺) and anions (A^-). The internal pH of the microbe decreases, which the bacteria are unable to tolerate.

A specific H⁺-ATPase pump acts to bring the pH inside the bacteria level. This phenomenon consumes energy and eventually stops the growth of the bacteria or even kills them. The lowering of pH also suppresses the enzymes, e.g. decarboxylases and catalyses, inhibits glycolysis, prevents active transport and interferes with signal transduction. The anionic (A^-) part of the acid is trapped inside the bacteria and becomes toxic by creating anionic osmotic problems for the bacteria. Thus, the antibacterial effects of organic acids work though:

modification of internal pH;

inhibition of fundamental metabolic functions;

accumulation of toxic anions;

disruption of the cellular membrane.

Non-pH-sensitive bacteria

The non-pH-sensitive bacteria are able to tolerate a large differential between internal and external pH. At a low internal pH, organic acids re-appear in a non-dissociated form and exit the bacteria. Equilibrium is created and the bacteria do not suffer.

A case of study

A broiler breeder operation was experiencing a problem of low hatchability, which was identified as being due largely to poor eggshell quality. The most common defects were thin, broken and rough shells.

On chemical analysis, the drinking water on the farm was found to be alkaline and hard (table 6).

Table 6:

Chemical analysis of drinking water

Ruling out other possible reasons for the poor eggshell quality, we focussed on the water alkalinity and hardness. To counter the problem, we decided to use a gut acidifier (Acid LAC; Kemin; at 0,25 %). The product is a synergistic combination of organic acids and their salts to regulate gut pH. Laboratory analysis of the feed, untreated and with two levels of the acidifier, is shown in table 7.

Table 7:

Analysis of the feed

Within one week, the proportions of thin-, broken- and rough-shelled eggs began to fall. The results after 5 weeks of treatment are shown in table 8.

Table 8:

Comparative data before and after use of gut acidifiers

Conclusion

Because of their pH-reducing and antimicrobial effects, gut acidifiers appear to be good substitutes for AGPs where the use of the latter is banned or severely restricted.

In a recent case study, the alkalinity of both the feed and drinking water appeared to contribute to poor eggshell quality in a broiler breeder flock. A gut acidifier looked promising in tackling this problem.

Dr Avinash Dhawale,

Farm Manager,

Diamond Hatcheries Private Ltd,

Hyderabad, India

Poultry International, Vol. 44, # 4, april 2005, Watt Publishing Co., U.S.A.

www.wattpoultry.com