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Effect of Protein Level and Penicillin on Growth and Intestinal Flora of Chickens

Five Figures

Departments of Bacteriology and Poultry Husbandry, Ontario Agricultural College, Guelph, Canada

Penicillin caused an increase in weight and improvement in feed efficiency of chickens fed diets containing 17, 20, 23 and 26% protein. While penicillin did not appear to change the protein requirement, the evidence suggests that the antibiotic enhanced protein utilization.

A considerable reduction in the pH of the cecal contents of chickens resulted from the addition of penicillin to diets containing 17, 20, 23 and 26% protein. The extent of the reduction increased with increasing protein up to the 23% level. The alterations in pH appeared to be associated with the increase in numbers of lactobacillus, aciduric, anaerobic and microaerphilic, aerobic and coliform types of organisms.

The increase in coliforms was due to the presence of an atypical strain of E. coli. Enterococci counts were reduced by the addition of penicillin, the reduction being similar at all levels of protein. Penicillin effected a marked increase in the size of the ceca with all levels of protein.

In in vitro respiration studies using the various diets as substrate and the specific organisms as inoculant, penicillin enhanced the O₂ uptake of lactobacillus and the aciduric type of organisms.

With diets containing levels of protein up to 23% a relatively close correlation was noted between increased growth of the birds and elevation in the numbers of certain microorganisms. It is suggested that the increase in numbers of these microorganisms or the resultant increase in the size of the ceca may be factors in explaining the mechanism of growth response to the antibiotic. On the other hand, the decrease in pH may cause increased activity of the intestinal villi or more complete digestion, and thus play a role in the growth response mechanism.