Human salmonellosis outbreaks have been linked to contamination of animal feeds. Thus, it is crucial to employ sensitive Salmonella detection methods for animal feeds.

Based on a review of the literature, Nelson A. Cox of the Richard B. Russell Agricultural Research Center in Athens, Georgia and co-authors there and with Anitox Corp. found that Salmonella sustains acid injury at about pH 4.0 to 5.0. Low pH can also alter the metabolism of Salmonella, hampering the identification process.

Because most Salmonella bacteria present in feed and feed ingredients are already stressed by desiccation and heat during conditioning, pelleting, or both, exposure to low pH may kill or severely injure these cells and result in contamination going undetected, the researchers reported recently in Journal of Applied Poultry Research.

Five grams of seven different feed ingredients and five types of feed were added to 45mL of buffered peptone water (BPW), lactose broth (LB), minimal salts medium (M-9), or universal pre-enrichment (UP). Media were incubated at 37°C for 18, 24 and 48 hours and the pH of the media was determined.

In the case of cereal grains, in general, the initial pH of the pre-enrichment media ranged from 6.1 to 7.2. Distillers dried grains with solubles in lactose broth was the exception to this trend (pH 4.5). After 18 hours of incubation, the pH of the media had decreased to a pH range of 4.0 to 6.1.

The pre-enrichment media behaved in a similar fashion when testing oilseed meals, canola and soya. i.e. initial pH range was 6.2 to 7.2 and decreased to pH 4.2 to 5.4 for soy and pH 4.4 to 6.0 for canola).

With these ingredients, the buffering capacities of BPW, LB, and M-9 were similar. There was more variation in the pH of the pre-enrichment media among the different poultry feed types.

The media from broiler and layer feeds had an initial pH of 6.2 to 7.0, whereas turkey feed was 6.4 to 7.1. After 18 to 48 hours incubation in broiler and in layer feed, the pH was 3.9 to 5.6, compared with pH values of 4.5 to 6.8 for turkey feed.

For feed, the buffering capacities of BPW, LB and M-9 were similar.

After incubation in commonly used pre-enrichment media, Cox and co-authors concluded that mixed feeds and feed ingredients can reach a pH that may kill or injure Salmonella. In light of these data, they add, detection methods for Salmonella in feed and feed ingredients may need to be re-evaluated.

Reference

^{Cox N.A., J.A. Cason, R.J. Buhr, K.E. Richardson, L.J. Richardson, L.L. Rigsby and P.J. Fedorka-Cray. 2013. Variations in preenrichment pH of poultry feed and feed ingredients after incubation periods up to 48 hours. J. Appl. Poult. Res. 22(2):190-195. doi: 10.3382/japr.2012-00552}

July 2013