In this study, total of 240 samples comprising 160 normal and 80 mastitic milk samples were processed for isolation and identification. On the basis of morphological, growth and biochemical characteristics, 29.58% and 9.16% isolates were presumed as
E. coli and
Klebsiella spp., respectively (Table 3). Thus, larger proportion of these was confirmed as (22.92%)
E. coli while 7.92% as
Klebsiella spp. through PCR amplification (Fig 2, 3 and Table 3). These findings were found in agreement with the reports of previous worker
(Ibrahim et al., 2018; Geser et al., 2012). Comparatively higher isolation rate of
E coli in this study can be attributed to high prevalence of
E. coli in the GIT flora.
To confirm the identity of isolates, gene sequencing was done and uidA gene sequence data indicated that
E. coli isolates with Accession No: MW353603 and Accession No: MW353604 possessed great similarities (99.82%) to
E. coli strain K-12 (LR881938.1),
E. coli ST18 strain (CP060709.1),
E. coli O68:H12 strain (CP061758.1) and
E. coli strain EC93 (CP061329.1).
Klebsiella isolates with Accession No: MW346043 and Accession No: MW346044 were identified as
Klebsiella pneumoniae strain K1and
Klebsiella pneumoniae strain K3, respectively. These isolates possessed 100% similarities to
Klebsiella pneumoniae strain NU-CRE047 (CP025037.1),
Klebsiella pneumoniae strain G17SC 16S (KX610833.1) and
Klebsiella pneumoniae strain WP5-S18-ESBL-06 (AP022157.1) on gene sequencing of 16S rRNA.
To study the occurrence of carbapenemase producers in milk, total 74 confirmed isolates were selected. On preliminary screening, 21.62% isolates were presumed as carbapenemase producers, out of which 17.56% isolates were found positive using DDST and 16.21% using MBL-E-strip test (Fig 5). There was little difference in the sensitivity of both of the test used for detection of carbapenemase producers and this observation corroborated with the findings of
Gupta et al., (2013) and
Bora et al., (2014). The finding of present study revealed higher frequency of carbapenemase producers in
Klebsiella spp. (26.31%) than in
E. coli (12.72%), which has been reported as major carbapenemase producer in previous studies also
(Gupta et al., 2013 and
Bora et al., 2014). The occurrence of carbapenemase producers was 10.0% (5.0%
E. coli and 5.0%
Klebsiella spp.) in cattle’s mastitic milk, while 20.0% (12.5%
E. coli and 7.5%
Klebsiella spp.) in buffaloes’ mastitic milk. The overall prevalence of carbapenemase producers was 5.0% with 2.91%
E. coli and 2.08%
Klebsiella spp. None of the carbapenemase producers was detected in normal milk (Table 4). There is scanty information on isolation of carbapenemase producers from milk both in India and abroad. In India,
Ghatak et al. (2013) have reported very low percentage of carbapenemase positive
E. coli in mastitic milk from NEH region while
Diab et al., (2017) reported 1.6%
Klebsiellae in raw milk samples from Lebanon. Similar to our finding, low percentage of carbapenemase positive
E. coli has also been reported by
Braun et al., (2016), Webb et al., (2016) and
Nirupama et al., (2018) from faecal sample of different species. However, some of the workers have reported higher prevalence of carbapenemase producers among faecal samples ranging between 21.64%- 29.03% from different parts of India
(Gupta et al., 2019; Murugan et al., 2019; Pruthvishree et al., 2017). These differences in findings of various co-workers may be due to variations in source, type of samples, animal husbandry practices in geographical locations.
Genotypic analysis of 12 phenotypically confirmed carbapenem resistant isolates (07
E. coli and 05
Klebsiella spp.) was done by targeting carbapenem genes
viz. bla-NDM,
bla-KPC and
bla-OXA-48 (Fig 7, 8, 9). The overall gene distribution study showed 42.85% occurrence of
bla-NDM and
bla-KPC in
E. coli isolates while 20.0% of
bla-OXA-48 and
bla-KPC
in Klebsiella spp.
E. coli isolates of cattle harboured both
bla-NDM and
bla-KPC gene (14.28% each) whereas
Klebsiella spp. harboured only
bla-OXA-48 gene (20.0%) in mastitic milk. In case of Buffalo,
E. coli isolates exhibited
bla-NDM and
bla-KPC gene (28.57% each), while
Klebsiella spp. isolates revealed only
bla-KPC gene (20.0%) in mastitic milk (Table 5). It was notable in this study that
bla-KPC gene was present in both
E. coli and
Klebsiella spp. isolates and
bla-OXA-48 gene was present only in one isolates of
Klebsiella spp. Although occurrence of these genes in animals is very low but it is very significant from public health point of view, as these are increasingly being reported from nosocomial infection in which
Klebsiella spp. is most commonly implicated. To the best of our knowledge, this is the first report of
bla-KPC and
bla-OXA-48 gene detection in mastitic milk of bovine from India. Earlier to our study, only
Ghatak et al., (2013) have reported
bla-NDM gene in mastitic milk. The occurrence of such genes in milk samples of bovine may be presumed due to horizontal transfer by unclean hands of workers, water supplied for washing of udder and utensils or poor environmental condition. These carbapenemase genes have also been reported in previous studies like
bla-NDM-1 and
bla-OXA-48 genes in piglets
(Pruthvishree et al., 2017; Nirupama et al., 2018), bla-VIM in calves
(Murugan et al., 2019), bla-OXA-48 in cattle faeces
(Braun et al., 2016) and in milk
(Diab et al., 2017) from India and abroad. The detection of these genes in milk sample isolates is a matter of serious concern, because it is very easier for such isolates to enter the food chain leading to serious food-borne illnesses.
Antimicrobial resistance is currently a serious global problem that has received the attention of larger scientific community. Antimicrobial susceptibility testing (AST) of carbapenemase positive isolates revealed that all isolates of
E. coli and
Klebsiella spp. were (80%-100%) resistant to imipenem, meropenem cefotaxime, cefpodoxime, ceftazidime, ceftriazone, aztreonam and ampicillin. The plausible factors for high degree of resistance to these antibiotics may be due to persistent antibiotic pressure or acquired from environmental sources or farm workers. Susceptibility pattern of these isolates differed with different classes of non-β-lactam antibiotics except polymyxin-B for which both isolates were found 100% susceptible (Fig 6).
E. coli isolates were found 85.0% to 100% sensitive against gentamicin and chloramphenicol, respectively while
Klebsiella spp. was found 80% to 100% sensitive against chloramphenicol and amikacin. There is abundant evidence to corroborate the emergence of resistance against 3rd generation cephalosporins and ampicillin in India and abroad for both
E. coli and
Klebsiella spp. isolated from milk of bovine origin
(Batabyal et al., 2018; Ghatak et al., 2019; Badri et al., 2017; Ibrahim et al., 2018). In this study all carbapenemase producing isolates were found to be multi-drug resistant (MDR)
i.e., resistant to three or more classes of antimicrobials. This finding was in concordance with the finding of
Bora et al., (2014). The occurrence of MDR in this study highlighted a potential threat by limiting the therapeutic options.