Variability in Growth Behavior of Carnobacterium Isolates in Medium with Low Initial pH

Introduction: Carnobacterium spp. are frequently isolated from vacuum-packaged (VP) beef, the pH of which is relatively low, around 5–6. Previous studies have demonstrated that the predominance of certain Carnobacterium strains in the final microflora may be linked to the storage life of VP beef. Information on the growth behavior of Carnobacteria would then be helpful to better understand the development of bacterial community structure on VP beef.

Purpose: To evaluate the growth behavior of Carnobacterium isolates at pH conditions relevant to VP beef.

Methods: The growth of 36 Carnobacterium isolates from VP meat cuts obtained from three abattoirs (A, B, and C) were determined in broth medium with initial pH 5.4 at 30^oC via measurement of optical density at 600 nm (OD₆₀₀). F-test was performed to determine whether the growth kinetics, including detection time, growth rate, and ΔOD (the difference of OD₆₀₀ between initial and stationary growth phase), were different among abattoirs, species, strains, and isolates. If P < 0.05, t-test was further used for pairwise comparison. The minimum pH (pHmin) allowing growth in BHI was further determined for each isolate.

Results: In medium with initial pH 5.4, detection time for strain NFU35 was significantly (P < 0.05) shorter than MMF-23, MFPB14D06-04, and G117. G117 showed the smallest ΔOD, followed by MFPB, NFU35, and MMF-23. The overall difference in growth kinetics among species was not significant, and the average detection time for isolates from abattoir A was longer than the other two abattoirs due to the presence of strain G117. The pHmin of G117 was 5, higher than the other isolates (4.4–4.8).

Significance: The different resistance of Carnobacterium strains to relatively low pH is likely one of the reasons resulting in difference in the microbial structure, which in turn lead to different storage lives of VP beef.