A comparison of the sensitivity and selectivity of MYPA and BYPTA
for various strains is shown in Table 1. All B. cereus group strains were
able to grow on MYPA and BYPTA, indicating that these selective
media have identical sensitivity. However, the selectivity of BYPTA
for pure cultures of non-B. cereus strains was greater than that of
MYPA. Significantly fewer Gram-negative and Gram-positive non-B.
cereus strains were able to grow on BYPTA (Gram-negative, 6/25;
Gram-positive, 14/26; p b 0.05) than on MYPA (Gram-negative, 21/25;
Gram-positive, 24/26; p b 0.05) (Table 1). It appears that the growth
of non-B. cereus species was further inhibited by the addition of trimethoprim
to selective media. Although trimethoprim has been reported
to only inhibit Gram-positive bacteria, the growth of Gram-negative
non-B. cereus was also inhibited by the addition of the antibiotic to selective
media. Growth inhibition of non-B. cereus species on BYPTA plates
may be due to the synergistic effects of trimethoprim and polymyxin B
against Gram-negative bacteria, as reported by Rosenblatt and Stewart
(1974). Thus, trimethoprim could potentially be an effective, selective
agent in media, used to detect B. cereus by preventing both Grampositive
and -negative competing microflora from growing (Chon et al.,
2012; Fricker et al., 2008).