Comparative and Veterinary Pharmacology

antimicrobial agent (Fig.3a, b). It can be seen that in the first case, the minimum
bactericidal concentration (MBC) distributions largely follow those of the MIC. In
contrast, in the second example, the median MIC values are similar to that of the
first (with the MIC values being only one dilution apart), but the MBC values
have markedly different distributions. What is most disconcerting is that this kind
of difference also has been observed within a bacterial species where strains
exhibiting similar MIC distributions can exhibit markedly different rates of killing
(P Silley, personal observation). Findings of this kind suggest the need to evaluate
MBC, in addition to (or in lieu of) MIC, as a PD parameter.

Distribution of MICs/MBCs for Species A (Gram -ve)

0.5 1 2 4 8 16 32 64 128 256

Concentration (mcg/mL)

n

MIC

MBC

0

2

4

6

8

10

12

a

b

Distribution of MICs/MBCs for Species B (Gram +ve)

0

2

4

6

8

10

12

14

16

18

20

0.5 1 2 4 8 16 32 64 128 256

Concentration mcg/mL

n

MIC

MBC

Fig. 3(a,b) Comparison of the relationship between MIC and MBC for two microbial species:
(a) a Gram-negative (ve) organism; (b) a Gram-positive (+ve) organism

248 M. Martinez and P. Silley