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The indigenous gastrointestinal microflora acts as a barrier
against incoming pathogens and overgrowth of opportunistic
microorganisms already present in the gut. Alterations in the
microbiota can create a window for opportunistic pathogens lead-
ing to possible overgrowth of resistant strains [ 11 – 15 ]. One of the
most dramatic modifications to the gut community is antibiotic
treatment [ 11 , 12 ]. Overgrowth and establishment of resistant
strains is not seen to the same degree in individuals not under the
influence of antibiotic [ 16 – 19 ]. An association of antibiotics and
presence of resistance has been described [ 16 – 20 ].
Mice are used extensively in animal experiments to study the
impact on intestinal colonization of different bacterial species with
relatively low cost and with good reproducibility [ 21 , 22 ].
Furthermore, the intestinal flora of laboratory mice and men are
comparable, which is why mice are often the first mammal used to
explore the association between intestinal microbiome, health and
disease [ 23 ]. Mouse models are also used to investigate bacterial
colonization and studies often include the administration of antibi-
otics prior to inoculation of the bacteria of interest [ 23 – 29 ].
Mouse intestinal colonization of Gram-negative bacteria has
successfully been determined in several studies. Yet most models
use elimination of resident facultative bacteria prior to inoculation
[ 24 , 26 , 30 – 32 ]. As such, mice treated with streptomycin have
been shown to be susceptible to enteric infection [ 11 ]. Additionally,
previous mice-studies have shown that exposure to sub-therapeutic
concentrations of penicillin, vancomycin, penicillin and vancomy-
cin, or chlortetracycline produced elevated ratios of Firmicutes to
Bacteroidetes. In addition, treatment with antibiotics of broad
spectrum of activity, or impact on the anaerobic flora, has been
shown to reduce the Bacteroidetes population [ 11 , 12 , 31 ].
Furthermore, Perez et al. studied the effect of subcutaneous treat-
ment on the indigenous intestinal microflora of mice and investi-
gated the effect on colonization by a KPC-producing Klebsiella
strain (KPC-Kp strain) [ 31 ]. Their findings indicated that the
anaerobic effect of antibiotics promoted the establishment of the
KPC-Kp strain—provided that the antibiotic had no effect on the
strain carrying resistance genes. Thus, antibiotics with limited
effect on the anaerobic flora might be less likely to promote the
colonization of multi-drug resistant Gram-negative bacteria [ 31 ].
Enterobacteriaceae, such as E. coli are able to colonize and sur-
vive in many different locations, including the human gastrointes-
tinal tract [ 9 , 33 ]. Moreover, E. coli is one of the organisms most
frequently found harboring genes coding for Extended-spectrum
beta-lactamases (ESBL) [ 3 , 8 , 34 ]. In recent years asymptomatic
carriage of antimicrobially resistant E. coli in humans has been
found in different parts of the world, with low to modest carrier
rates of 6–13% and high carrier rates of 50–63% [ 35 – 38 ].Frederik Boëtius Hertz et al.