may be genetically identical, the individual cells can express a wide variety of
traits, including differences in basic metabolic activity, antibiotic tolerance spore
formation, and the secretion of extracellular polymers. In fact, it is the secreted
polymers that are a defining feature of biofilms.
Biofilms can include multiple bacterial species and they confer a survival
advantage in several ways, including (Jefferson 2004 ):
l Protection against sheer forces and adverse conditions such as nutrient depriva-
tion, changes in oxygen, changes in pH, or the presence of antibodies
l The facilitation of bacterial colonisation
l The integration of bacterial communal activities, thereby allowing for the
division of labour to facilitate bacterial protection and metabolic integrity
This phenotypic transformation generally occurs deep within the biofilm and
represents a slow or non-growing phase. The slow or non-growing condition leads
to a reduced (or absent) sensitivity to certain compounds, particularly those that are
dependent upon internal synthetic mechanisms, such as theb-lactams (Tanaka et al.
1999 ). For example, the reduced susceptibility tob-lactams is related to a dimin-
ished expression of penicillin-binding proteins and a decrease in the drug-induced
inhibition of transpeptidases (Gilberta and Brown 1998 ). Such findings have led to
the term “drug indifference” (Jayaraman 2008 ). However, biofilm-associated bac-
teria are not “resistant” to antimicrobials, and ultimately they revert back to a
sensitive state once they convert back to the planktonic form. Furthermore, even
non-growing biofilm cells remain sensitive to the fluoroquinolones or to drugs that
interact with and disrupt cell membranes (Tanaka et al. 1999 ; Jayaraman 2008 ;
Smith et al. 2009 ). It has been shown that C-8-methoxy fluoroquinolones are
superior to other fluoroquinolones in their ability to kill under anaerobic conditions
and they are also particularly effective even when protein synthesis is blocked
(Malik et al. 2007 ). This may be a function of substitution at the C-8 position as
pradofloxacin, with a C8-cyano grouping, has been shown to be active in the
absence of protein synthesis (Ko ̈rber et al. 2002 ). In vitro data illustrating the
relationship between drug class, bacterial species, growth phase, and killing activ-
ity, is shown in Fig. 4 (based upon data from Eng et al. 1991 ).
Biofilm formation is now recognised to represent an important component of
most human bacterial diseases including endocarditis, osteomyelitis, dental decay,
middle ear infections, tuberculosis, medical device infections, and cystic fibrosis
(Donlan and Costerton 2002 ). An example of a biofilm that has relevance in
veterinary medicine comprises recurrent urinary tract (E. coli) infections (Opal
2007 ; Ball et al. 2008 ; Soto et al. 2006 ) and bovine (S. aureus) mastitis (Melchior
et al. 2009 ).
In addition to biofilms, there are many circumstances in which relapses occur
because surviving bacterial cells appear to have the capacity to resuscitate and
divide once the concentration of the antimicrobial drug decreases below some
critical value. Thus, in addition to typical slow growing or non-growing cells of
the biofilm, there appear to be some bacteria that form “persister cells”. These cells
can survive assault from all antimicrobial agents (including fluoroquinolones and
Antimicrobial Drug Resistance 251