FBA Issue 31: March / April 2010

 

 

 

 

 

Prevention of disease transmission and enhancement of growth and feed efficiency are critical factors in modern animal production (Mroz, 2005). When pathogenic bacteria contaminate feed, it becomes a potential disease transmission route to human and livestock populations. This makes it of great concern to producers and consumers alike (Crump et al., 2002; WHO, 2006). Food - producing livestock (cattle, chickens, pigs, and turkeys) are the main reservoirs for many of these microorganisms, which include non-Typhi serotypes of Salmonella enterica, Campylobacter species, Shiga toxin producing strains of Escherichia coli, and Yersenia enterocolitica (Mead et al., 1999).

 

The microflora found in feed materials comes from a variety of ecological niches including soil, livestock gastrointestinal (GI) tracts and residues left over from earlier inhabitants of swine feed lots. The GI tract pathogens can be introduced into food chain by animals defecating in a farm environment or by fertilisation of crops with manures (Maciorowski et al., 2007). This consequentially makes feed a carrier for animal and human pathogens. 

 

Feed materials may be inoculated with microorganisms, mostly bacteria and fungi, at any time during growing, harvesting, processing and storage. Counts of microorganisms vary depending on the function of materials, location of its origin and climatic conditions (D'Mello 2006). It is known that microfloral growth is dependent on moisture, pH value, temperature and composition of feed materials (Maciorowski et al., 2007). For example, the optimal temperature for E. coli O157:H7 is 37^°C, with a minimum of 7 - 8^°C and a maximum of 46^°C. The optimal pH is between 6 and 7, though it tolerates pH ranges between 4.4 to 9.0. The E. coli O157:H7 doubles in number approximately every 0.4 hours at optimal temperatures and pH value.

 

Some microorganisms including E. coli may adapt to conditions without water and can actively grow in stored feed. Some authors reported that grains and oilseed crops possess a diverse microflora, with populations ranging from 5x103 to 1.6x108 colony-forming units (CFU)/g. Such microbes are highly resistant to low moisture conditions (Multon, 1988; Richard-Molard, 1988).

 

The orientation values of mesophilic bacteria, including Listeria monocytogenes, Pesudomonas maltophilia, Thiobacillus novellus, Staphylococcus aureus, Streptococcus pyrogenes, Streptococcus pneumoniae, E. coli and Clostridium kluyveri etc. for swine compound feed are shown in Table 1 (VDLUFA, 2007).

 

 

Experimentally, very low doses of E. coli O157:H7 may result in colonisation of some piglets. Once some piglets are colonized, they may amplify E. coli O157:H7 and transmit it to other piglets via contact. Enterotoxigenic E. coli strains are a major cause of diarrhea and death in neonatal and newly weaned pigs. Enterotoxigenic E. coli adhere to the small intestinal microvilli and produce enterotoxins that act locally on enterocytes. This action results in hypersecretion of water and electrolytes, and reduced absorption (Amezcua et al., 2002). 

 

Heat treatment, usually during conditioning, pelleting or extrusion has been shown to be an effective way to reduce microbial loads in feed materials and compound feed. Reduction of the bacterial contamination by heat is dependent on the temperature and treatment time.

 

At the same time, these methods do not prevent a recontamination of feed materials and compound feed afterwards (Çelik et al. 2003; Maciorowski et al., 2007, WHO, 2006). Fortunately, dietary acidification with organic acids has been shown to contribute to environmental hygiene, thereby preventing feed raw materials and compound feed from microbial and fungal deterioration.

 

Moreover, constant treatment with organic acids has a residual protective effect in feed, which helps to reduce its recontamination along with the related contamination of milling and feeding equipment. Supplementation of organic acids in feed tends to decrease the feed pH and buffer capacity, thereby preventing undesirable microbe growth.

 

However, for each acid, its specific inhibiting effect on bacteria, yeast and mould has to be considered when recommendations for feed supplementation are made. For example, some organic acids, like formic and propionic acids, have broader antimicrobial activities and can be effective against bacteria and fungi, including yeast. 

 

 

Dietary acidification is important for creating unfavorable conditions for microorganisms, reducing pH and stimulation of GI tract enzymes. Optimum pH is needed for enzyme activation. For example, pepsinogens are rapidly activated at pH 2, but act very slow at pH 4. Pepsin has its optimum between pH 2 and 3.6, and remains inactive at pH 6 (Kidder and Manners, 1978). Due to insufficient production of HCl, pancreatic enzymes, and sudden changes in feed consistency and intake, piglets have limited digestive capacity and absorption at weaning.

 

Moreover, the stress associated with weaning is known to disturb intestinal microflora. Various studies show that acidification of the diets decreased pH-value in feed and consequentially reduced the coliform and E. coli counts along the intestinal tract, thereby decreasing scouring and mortality of piglets.

 

It has been shown that acid conditions favor the growth of lactobacilli in the stomach, which possibly inhibits the proliferation of E. coli and produces lactic acid and other metabolites which lower the pH and inhibit E. coli. The reported pH levels of the swine diets in these studies range from 4.36 to 5.79 (Kluge et al., 2006; Partanen et al. 2007; Mroz 2008). Dietary acidification by a mixture of organic acids decreased the pH value in swine diets by 0.15 to 0.98 pH units.

 

The decrease in pH values was dependent on the inclusion levels of organic acids, which varied from 0.5 to 3%, and were impacted by diet composition. This was in agreement with a recent study, where a blend of formic and propionic acids (Biotronic^® SE forte, BIOMIN, Austria) at an inclusion level of 0.3% reduced the pH by 0.11 pH units in both starter and grower diets. A higher inclusion level of 0.5% of the same acid blend reduced pH of the diet by 0.23 and 0.21 pH units in both starter and grower diets, respectively. The pH and B - values of starter and grower feed are shown in Table 2.