A CONVERSION ON THE CULTURE PLATE OF RICHARD JULIUS PETRI AN ASSESSMENT IN RELATION TO THE STANDARD DISC DIFFUSION TECHNIQUE

Mohammad Farid  A.R. EL BAL'A

Several methods that consider the rate, extent, or reversibility of bacterial damage as criteria useful in the assessment of activity of different agents have been described. These methods have often relied on viable counts, turbidimetric monitoring, or surface growth transfer techniques, these differed from recommended susceptibility test methods. The need for an approach, based on standard methods, yet capable of assessing the rates, extent and reversibility of drug induced bacterial damage was recognized.

The Petri plate was converted with the intent of deriving a second functional agar surface. The flat closed bottom was replaced with a wire mesh. The set‑up consisted of: (1) a sieve culture plate; (2) a compartment, to close the mesh area and permit pouring of molten agar media into the plate; (3) a washing flask, to contain the fluid during the exchange process; (4) a holder, to suspend different plate sizes in position for exchange; (5) a magnetic stirrer that could operate for extended time periods without over‑heating and: (6) a mechanism, to safeguard against flooding of the culture surface when the fluid reaches the bottom of the sieve.

Antimicrobial removal  from diffusion zones was assessed by the absence of surface inhibition as compared to plates that did not undergo exchange with Mueller-Hinton broth. A comparative assessments of the standard disc diffusion zones resulted in a correlation coefficient of +0.989 for zone diameter pairs determined on the devised plate and commercially obtained Petri dishes. Inhibition zone interpretations agreed in 96.4% of 1260 paired determinations.

Changes occurring in standard inhibition zones following 24 hrs of agar‑liquid exchange in Mueller-Hinton broth, were grouped into five patterns, reflecting the recovery of tested populations in relation to antimicrobial gradients, produced by diffusion. Attempts were made to correlate observed recovery patterns to agar dilution, broth micro‑dilution MIC, and MBC. Zones resulting in no change observed following the exchange process consistently demonstrated ratios of MBC 99.99% to MIC within 1 to 2 folds. Other patterns yielded differing mean inhibitory and lethal concentrations. Susceptible zones of different genera (Escherichia, Klebsiell.a, Enterobacter, Serratia, Acinetobacter, and Pseudomonas)) yield differing totals for each of the five recovery patterns. Isolate to isolate variability in the extent and type of bacterial recovery was observed.

Staphylococcus aureus isolates, resistant to methicillin, and demonstrating susceptibility to cephalothin by the disc diffusion, showed a total disappearance of the previous inhibition zone following 24 hrs. of exchange.

The determination of bacterial recovery, in susceptible inhibition zone may lead to an extra test parameter useful in the comparative assessment of antimicrobial agents. The clinical significance of the recovery patterns observed in susceptible inhibition zones remains to be determined. Future recommendations and potential applications of the "Sieve Culture System" (SCS) are also outlined.