Ke. Aldridge et Dd. Schiro, ANAEROBIC SUSCEPTIBILITY TESTING - SLIGHT DIFFERENCES IN INOCULUM SIZE CAN MAKE A DIFFERENCE IN MINIMUM INHIBITORY CONCENTRATIONS, Diagnostic microbiology and infectious disease, 18(3), 1994, pp. 191-195
In the present study, we compared the actual inoculum density from ino
culated broth microdilution wells to the targeted inoculum size (10(5)
CFU/well) when the inoculum was prepared using a McFarland nephelomet
er to achieve the standard density. Three target inoculum sizes (10(5)
, 5 x 10(5), and 10(6) CFU/well) were used to compare the effect of sl
ight inoculum size increases of both ATCC and clinical strains of anae
robes on MICs of various antimicrobials. Actual colony counts of Bacte
roides fragilis, Bacteroides thetaiotaomicron, Eubacterium lentum, and
Veillonella parvula ranged from 0.7 x 10(5) to 1.4 x 10(5) CFU/well.
As the inoculum size rose above the desired 10(5) CFU/well level, the
MICs of certain antimicrobials became elevated. Ceftizoxime, cefotaxim
e, and ceftriaxone MICs rose 4- to 16-fold with as little as 0.5 log(1
0) increase in inoculum size. Other increases were also noted with E.
lentum and Clostridium perfringens, but were primarily between the low
and high inoculum sizes. Results with cefoxitin, cefotetan, mezlocill
in, and imipenem did not show an appreciable increased inoculum effect
. This study demonstrates that the variation in organism size among an
aerobes (both between species and within species) does make a differen
ce in actual inoculum size and certain anaerobes may require special a
djustment to ensure proper MIC results from susceptibility testing.