Chromagar Staph aureus Versus Blood Agar and Mannitol Salt Agar ...

Egyptian Journal of Medical Microbiology, January 2007 Vol. 16, No. 1base (Acumedia, Baltimore, Md.)supplemented with 5% human blood (bloodagar) and mannitol-salt agar (Oxoid-CM85),were prepared according to the manufacturer'sinstructions.Clinical samples:Suppurative skin lesions of a total of105 pyoderma patients admitted to theoutpatient clinic of the Main UniversityHospital, Alexandria, Egypt, were included inthis study. Swabs were used to collectsuppurative exudates from skin lesions, thenimmediately delivered to the lab forprocessing. Each swab was inoculated andspread onto each of the three types of media(BA, MSA and CAS). All plates wereaerobically incubated at 37°C, BA for 24 h (10), CAS for 48 h (according to manufacturerinstructions) and MSA for 48 h. (10)Identification of S. aureus.Golden yellow hemolytic or nonhemolytic colonies on blood agar, yellowmannitol fermenting colonies on MSA andmauve colonies on CAS were regarded aspresumptive S. aureus and were confirmedwith Gram stain reactions, reactions tocatalase (3% [wt/vol] hydrogen peroxide,reactions to slide and tube coagulase (rabbitplasma), and DNase activity were used foridentification. (10) RESULTSEighty three confirmed strains of S.aureus were isolated (from 105 swabs takenfrom suppurative skin lesions) on one or moreagar culture media within 48 h of incubation.A total of 83 strains were isolated onBA after 24 h of incubation.On CSA, 83 strains were recovered asmauve colonies after 24 h and no furthergrowth was detected after incubation for 48 h.One of these mauve colony was not provedto be S. aureus (false positive). These resultswere associated with sensitivity of 98.79 %and a specificity of 95.45 %. (Table 1)As regards MSA, 43 strains wererecovered as yellow colonies after 24 h ofincubation compared with 62 strains afterincubation for 48 h. One yellow colony wasnot proved to be S. aureus (false positive).These results were associated with asensitivity of 50.60% and 73.49 % after 24and 48 h. respectively. (Table 2, 3)Table 1: • Chromagar Staph aureus versus blood agar for detection of S. aureus after 24incubation hours.Blood agarChromagartotalpositivenegativepositive 82 1 83negative 1 21 22total 83 22 105•After extending incubation to 48 hours mauve color of colonies start to fade away in most ofthe platesSensitivity = T+ve/T+ve+F-ve = 82/82+1= 98.79%Specificity = T-ve / T-ve+F+ve = 21/21+1= 95.45%Table 2: Mannitol salt agar versus blood agar for detection of S. aureus after 24incubation hours.Bl.agarMSAtotalpositivenegativepositive 42 41 83negative 1 21 22total 43 62 105Sensitivity = 42/42+41 = 50.60%Specificity = 21/21+1= 95.45%64