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Front. Cell. Infect. Microbiol.
Front. Cell. Infect. Microbiol.
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Abstract: |
A major increase of bacterial resistance to colistin, a last-resort treatment for severe
infections, was observed globally. Using colistin in livestock rearing is believed to be the
ground of mobilized colistin resistance (mcr) gene circulation and is of crucial concern to
public health. This study aimed to determine the frequency and virulence characteristics of
colistin-resistant Gram-negative bacteria from the milk of mastitic cows and raw
unpasteurized milk in Egypt. One hundred and seventeen strains belonging to
Enterobacteriaceae (n = 90), Pseudomonas aeruginosa (n = 10), and Aeromonas
hydrophila (n = 17) were screened for colistin resistance by antimicrobial susceptibility
testing. The genetic characteristics of colistin-resistant strains were investigated for mcr1–9 genes, phylogenetic groups, and virulence genes. Moreover, we evaluated four
commonly used biocides in dairy farms for teat disinfection toward colistin-resistant
strains. Multidrug-resistant (MDR) and extensive drug-resistant (XDR) phenotypes were
detected in 82.91% (97/117) and 3.42% (4/117) of the isolates, respectively. Of the 117
tested isolates, 61 (52.14%) were colistin resistant (MIC >2 mg/L), distributed as 24/70
(34.29%) from clinical mastitis, 10/11 (90.91%) from subclinical mastitis, and 27/36 (75%)
from raw milk. Of these 61 colistin-resistant isolates, 47 (19 from clinical mastitis, 8 from
subclinical mastitis, and 20 from raw milk) harbored plasmid-borne mcr genes. The mcr-1
gene was identified in 31.91%, mcr-2 in 29.79%, mcr-3 in 34.04%, and each of mcr-4
and mcr-7 in 2.13% of the colistin-resistant isolates. Among these isolates, 42.55% (20/
47) were E. coli, 21.28% (10/47) A. hydrophila, 19.12% (9/47) K. pneumoniae, and
17.02% (8/47) P. aeruginosa. This is the first report of mcr-3 and mcr-7 in P. aeruginosa.
Conjugation experiments using the broth-mating technique showed successful transfer of
colistin resistance to E. coli J53-recipient strain.
Different combinations of virulence genes
were observed among colistin-resistant isolates with almost all isolates harboring genes.
Hydrogen peroxide has the best efficiency against all bacterial isolates even at a low
concentration (10%). In conclusion, the dissemination of mobile colistin resistance mcr
gene and its variants between MDR- and XDR-virulent Gram-negative isolates from dairy
cattle confirms the spread of mcr genes at all levels; animals, humans, and environmental,
and heralds the penetration of the last-resort antimicrobial against MDR bacteria.
Consequently, a decision to ban colistin in food animals is urgently required to fight
XDR and MDR bacteria
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