Among other factors, infectious diseases are the real threat to the poultry development. Of the various infectious microorganisms Escherichia coli and Mycoplasma are of prime importance. These microorganisms persist for reasonably long periods in the flocks and there by lead to high economic losses through mortality, loss of weight and production. E. coli cannot readily infect the air sacs unless they are previously invaded by Mycoplasma gallisepticum alone or in combination of either infectious bronchitis or Newcastle disease virus. The present research was therefore designed to study the susceptibility of Mycoplasma infected birds towards E. coli as the average mortality rates observed in the occurrence of CRD as an individual disease was 8 per cent, whereas E. coli combined outbreaks recorded 15 per cent. In the present study, a total of 255 clinical specimens which included choanal swabs (244) from live and trachea (11) from dead birds affected with Mycoplasmosis were processed. Out of total 68 (12.15%) E. coli isolates recovered, 31 (45.58%) were associated with Mycoplasmosis and were recovered from choanal swabs. Recovered E. coli isolates subjected to AST by using different antibiotics like ampicillin (AMP), oxytetracycline (O), tylosine (TL), erythromycin (E), enrofloxacin (EX), norfloxacin (NX) and gentamicin (GEN). Multiple drug resistance was observed in all isolates of E. coli. Conventional characterization of E. coli isolates revealed, 12 (41.37%) isolates haemolytic, 26 (83.87%) invasive E. coli and 7 (22.58%) isolates positive for haemagglutination, respectively. In cvi gene PCR, 6 (19.32%) isolates yielded an amplification product of ~1181 bp. Out of 20 isolates tested which are resistant to ampicillin, 3 (15%) and 17 (85%) isolates yielded a product of ~403 bp in bla_TEM gene specific PCR using chromosomal and plasmid DNA, respectively. Plasmid profiling of the Escherichia coli isolates detected four different sizes of plasmids (~700 bp, ~1100bp, ~1650bp and >12kbp) with presence of >12kbp plasmid in all the isolates. The plasmids were distributed at random in the isolated E. coli strains. Correlation of presence of bla_TEM gene and plasmid profile was observed with resistance to ampicillin seen in AST.

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