In this study a total of 1200 quarter milk samples were selected from 300 cows from different localities
at El-Kaluobia governorate. The bacteriological examination revealed that a total of 125 isolates from
clinical and subclinical mastitic milk samples of which 57 S.aureus isolates were isolated from both
mastitic milk .with incidence of 45.6% of total isolates, 28 E.coli with 22.4%, 13 S.agalactia with 10.4%,
9 S. dysgalactia with 7.2%, 7 isolates of both S.uberis & coagulase negative staph (5.6%), 3 Klebsiella
sp with 2.4% and 1 Pseudomonas sp with 0.8%. Mean titer of anti S aureus specific IgA was higher in
milk of subclinical mastitic cases (1:845) than clinical one(1:290), while mean titer in milk of clinical
and subclinical mastitic cases much higher than in serum of clinical(1:54)and subclinical (1:41), while
mean titer of anti S. aureus specific IgM and IgG was much higher in serum of clinical(1:1382 , 1:868
for IgM&1:1267 , 1:634 for IgG ) than milk of clinical and subclinical mastitic cases (1:111 , 1: 53 for
IgM& 1: 302 , 1: 206 for IgG). Results of sensitivity tests of S.aureus against different antibiotic discs
revealed that 93%,91.2%,87.2% and 63.2%, were sensitive to Amoxicillin /Clavulanic acid,
Gentamycin, Cloxacillin, Sulphamethoxazole /Trimethoprim while87.7%,86.0%,84.2% and 82.5%of
isolates were resistant to Nalidixic acid, Streptomycin, Oxytetracycline and Penicillin-G.