Mycoplasma gallisepticum (MC) is a persistent, highly transmissiblechicken pathogen. It predisposes the birds to other infections such as E. coliyielding significant losses in performance and associated economics to allsectors of the poultry industry. In fact, one important feature ofMgallisepticum infection is that it can persist in the bird during all live, evenin the presence of the humoral antibodies.Therefore, the aim of this work is to perform a comparison between theconventional methods, conventional PCR and real time PCR in diagnosis ofsingle M gallisepticum infection and combined with E. coli in respiratorymanifested chickens, to apply a comparative study between natural andexperimental infections of chickens with M gallisepticum and/or E. coli andto evaluate the role of M. gallisepticum and E. coli virulence genes inSecond: 14 isolates from calf samples and 5 isolates from human specimens were studied for antimicrobial susceptibility patterns by disk diffusion tests using 14 antimicrobial disks.Third: the most five resistant and the most five sensitive E. coli isolates were subjected to multiplex PCR with primers specific to the most common virulence genes (Stx1, Intimin, F41, K99, STa and Stx2).