Purpose: This study compared the growth rates of Listeria spp. isolates from pastured poultry to other isolates from conventional poultry, humans, and plant matter.
Methods: Maximum specific growth rates were calculated from growth curves obtained using a micro plate reader, tube optical density (OD) readings, and plate counts (CFU/ml).
Results: Quadruplicate microplate reader experiments revealed that the ten pastured poultry strains had overall higher maximum growth rates (P<0.05) than the ten isolates from varied sources. L. innocua F6-840 and L. welshimeri F6-844, pastured poultry isolates, and L. monocytogenes EGD-e, a non-pastured poultry isolate, were then selected for growth measurements via plate counts and tube OD readings. There were differences in growth rates (P<0.05) between plate counts and tube OD readings compared to microplate reader data for L. innocua F6-840 and L. welshimeri F6-844. In all three strains, maximum growth rates from microplate reader data were less (P<0.05) than plate counts and tube OD readings. L. innocua F6-840 had higher growth rates (P<0.05) than L. welshimeri F6-844 and L. monocytogenes EGD-e, but this was observed only with microplate reader data. Plate count maximum growth rate from triplicate experiments for L. innocua F6-840 was 0.964 (±0.03), for L. welshimeri F6-844 the rate was 0.855 (± 0.12), and for L. monocytogenes EGD-e was 1.287 (± 0.31).
Significance: Understanding the physiological characteristics, such as maximum growth rates, of Listeria strains isolated from pastured poultry aids in developing more accurate risk assessments for this rearing method and furthers our understanding of their ecology.