Flasks of Tryptic Soy Broth, (TSB), unacidified (pH 7.2) or acidified
with HCl or lactic acid to pH 6.3 or 5.5 and samples of sterile pork f
at or muscle tissue, were inoculated with logarithmic phase cultures o
f a strain of Yersinia enterocolitica which had been isolated from a p
ork packing plant. The broth cultures were incubated at temperatures b
etween 2 and 25 degrees C, and growth rates determined from increases
in the optical densities at 600 nm. The tissue samples were incubated
at temperatures between -2.4 and 20.4 degrees C, and growth rates were
determined from increases in the viable counts. The organism grew wit
hout a lag phase in all broth cultures. At any temperature the rates o
f growth were lower in media of pH 6.3, and lower again in media of pH
5.5 than in unacidified broth; and growth rates were lower in media a
cidified with lactic acid than in media of the same pH acidified with
HCl. The data for each medium were well described by the regression li
ne of the plot of the square roots of growth rates against temperature
. The organism grew on fat tissue, of pH 6.3+/-0.3, without a lag phas
e at all temperatures, at rates comparable with the rates of growth in
unacidified TSB of pH 7.2; i.e. at rates faster than would be predict
ed for a medium of pH 6.3. The organism did not grow on muscle tissue
of pH 5.6+/-0.2, at temperatures less than or equal to 6 degrees C. At
higher temperatures, the organism grew on muscle tissue only after a
lag of 24 h or more at rates which were less than those that would be
predicted from the rates of growth in TSB acidified to pH 5.5 with lac
tic acid. It appears that models for the growth of Y. enterocolitica a
re likely to underestimate the growth on pork fat and to overestimate
the growth on pork muscle. Three previously published models would gen
erally predict much faster growth in broth cultures, under all the stu
died conditions of temperature and pH, than was observed for the strai
n used in this study. (C) 1995 Academic Press Limited