Clinical Resistance-Mic Values, Breakpoints, Phenotype and Outcome

CLINICAL  RESISTANCE—MIC  VALUES, BREAKPOINTS,  PHENOTYPE  AND  OUTCOME

The resistance mechanisms described in this chapter typically lead to an increase in MIC value, although it should be remembered that this does not always equate to clinical failure. If the MIC value remains below the breakpoint value, which can itself be difficult to determine, then the antibiotic will remain effective in the clinic. But such arguments assume that MIC values, which are typically determined when the isolate is growing in complex, sensitivity-test broth, equate with the sensitivity of the organism when growing in the many subtly different environments encountered during infection in vivo. Unfortunately, the antibiotic literature contains numerous examples of treatment failures despite apparent sensitivity in the test tube and this resistance is referred to as being phenotypic. In other words, resistance is a consequence of the adaptation of the organism to grow and survive within the in vivo environment, and subculture into conventional laboratory growth medium rarely shows the existence of resistant mutants. There are several key factors at play here, particularly slow/no growth, nutrient depletion and mode of growth. There are numerous papers showing a tendency for nutrient depletion, i.e. restricted or non-availability of an important nutrient, and slow/no growth to be associated with reduced susceptibility to antibiotics and biocides.

There is now increasing concern over the role played by microbial biofilms in infection (see Chapter 8). These include the well-known examples of medical devicerelated infections, such as those associated with artificial joints, prosthetic heart valves and catheters. Many chronic infections, not related to medical devices, are also due to bacteria either not growing and relatively dormant or growing slowly as biomasses or adherent biofilms on mucosal surfaces. A bacterial biofilm is typically defined as a population of cells growing as a consortium on a surface and enclosed in a complex exopolymer matrix. Commonly in the wider environment but less so in infections, the population is mixed and also of heterogeneous physiologies. Growth as a biofilm almost always leads to a large increase in resistance to antimicrobial agents, including antibiotics, biocides and preservatives, compared with cultures grown in suspension (planktonic) in conventional liquid media, but there is no generally agreed mechanism to account for this resistance. Although there is general acceptance that there are numerous planktonic phenotypes, many papers refer to ‘the biofilm phenotype’, implicitly assuming (wrongly) that there is only one. Those same parameters known to influence planktonic physiology and antibiotic susceptibility, including growth rate and/or specific nutrient limitation, also apply to biofilm physiology and antibiotic susceptibility. The general resistance of biofilms is clearly phenotypic. The well-characterized resistance mechanisms described above—lack of antibiotic penetration, inactivation, efflux and repair—make contributions in some circumstances. However, compelling evidence that they are uniquely responsible for biofilm resistance is lacking. Reduced growth rate probably has an involvement, particularly in that it is associated with responses to stress. During stress responses, key structures are protected and cellular processes close down to a state of dormancy, and it has been proposed that exceptional vegetative cell dormancy is the basic explanation of  biofilm resistance.