J Clin Pathol Qual Control.  1999 Jun;21(1):195-200.

Penicillin Susceptibility Screening of Streptococcus pneumoniae by Disk Diffusion with Blood Agar Plate; Is It Appropriate

• Kim SR
• Chang CL
• Jeong J
• Shin JH
• Kim HH
• Lee SH
• Son HC
• Kim SH

Abstract

BACKGROUND: Blood agar plates (BAP) have been used in penicillin susceptibility tests using the disk diffusion method with 1 microgram oxacillin disk in the country's laboratories, despite the recommendation of using Muller-Hinton agar supplemented with 5% sheep blood (MHSB). We compared results with those in the agar dilution minimal inhibitory concentration (MIC) method to confirm the applicability of BAP.
METHODS
With a total of 61 clinical pneumococcal isolates, penicillin MICs were tested using the agar dilution method, while oxacillin disk screening was performed using MHSB and BAP, respectively.
RESULTS
Penicillin MICs of the isolates ranged from 0.015 to 4 microgram/mL. with 9.8% susceptible, 37.7% intermediate, and 52.5% high-level resistant rates. In testing with MHSB, 83.3% (5/6) of susceptible strains were recovered with no false susceptibility. While testing with BAP, 83.3% of susceptible strains were also detected, with a 4.3% (1/23) of minor error. The inhibition zones were excellently correlated (r=0.9918, P<0.001), but the diameters were 2 mm larger in BAP than in MHSB.
CONCLUSIONS
It was reconfirmed that the use of MHSB is absolutely recommended in the screening of penicillin susceptibility of pneumococci by disk diffusion method with 1microgram oxacillin disk, and it was demonstrated that in the cases using BAP, the criteria of susceptibility should be reset as the inhibition zone equal to or larger than 22 mm instead of 20 mm for the prevention of falsely susceptible results.

MeSH Terms

Agar*
Diffusion*
Mass Screening*
Oxacillin
Penicillins*
Sheep
Streptococcus pneumoniae*
Streptococcus*
Agar
Oxacillin
Penicillins