Horizontal Gene Transfer in Planktonic and Biofilm Modes - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 78

3.4.1 Evidence for Transduction as an Important Means to Disseminate
Antibiotic Resistance
Brabban et al. ( 2005 ) summarized the current knowledge of the role of temperate
phages in the dissemination of antibiotic resistance. As transformation and conju-
gation are not common modes of HGT in Salmonella , phage-mediated transduction
has been suggested as the most important mode. The phages ES18 and PDT17 have
been shown to transduce antibiotic resistance genes in S. Typhimurium DT104, with
PDT17 having been found integrated into the genome of all strains of DT104 so far
studied (Schmieger and Schicklmaier 1999 ). This observation is consistent with the
fact that the core resistance genes in S. Typhimurium DT104 are chromosomally
encoded in a tight cluster as part of Salmonella genomic island I (43 kb), well within
the size that a phage could package and transduce (Schmieger and Schicklmaier
1999 ; Cloeckaert and Schwarz 2001 ; Brabban et al. 2005 ). Most of the other studied
strains of Salmonella carry complete prophages within their genomes, many of
which are capable of generalized transduction upon induction, spontaneous or
otherwise (Schicklmaier et al. 1999 ; Mmolawa et al. 2002 ; Bossi et al. 2003 ;
Brabban et al. 2005 ).
Brabban and coworkers also summarized recent observations on the induction of
phages from the lysogenic state to the lytic pathway. In particular, antibiotics
affecting DNA metabolism (such as the quinolones trimethoprim, norfloxacin,
and ciprofloxacin) can induce phages to leave their prophage state and reproduce
lytically, even at sub-inhibitory concentrations. Matsushiro and coworkers
observed a 1,000-fold increase in phage titers and a 60-fold increase of Shiga
toxin production within 6 h of in vitro ciprofloxacin exposure (Matsushiro
et al. 1999 ). Experiments using growth-promoting antibiotics typically used in
animal husbandry found that olaquindox and carbadox (both DNA targeting agents)
increased both phage and Stx production (K¨hler et al. 2000 ). This phenomenon is
not limited to E. coli O157:H7, but it has been also reported that antibiotic
resistance transfer occurred in V. cholerae at much higher efficiency when the
SOS response was induced by antibiotics (Beaber et al. 2004 ; Hastings et al. 2004 ).
Brabban et al. concluded that the use of antibiotics, whether therapeutically or as a
growth promoter, not only provides a selective pressure on bacterial populations
that favors resistant strains, but it also potentially increases the number of trans-
duction and lysogenic conversion events within the population (Brabban
et al. 2005 ).
The poultry industry faces a significant challenge in dealing with antibiotic
resistant strains of S. typhimurium , P. aeruginosa , and E. coli that infect and can
alter productivity of poultry flocks (Vandemaele et al. 2002 ; Walker et al. 2002 ;
Brabban et al. 2005 ). Thus, the continued presence of antibiotic resistant strains of
these common food-borne pathogens among poultry and other livestock coupled
with the prevalence of transducing phages in the gastrointestinal tract of these
animals is a matter of concern for public health officials worldwide (Brabban
et al. 2005 ). For example, multiple antibiotic resistance cassettes can be transduced