HiCombi™ Chocolate - MacConkey Agar Plate

Principle And Interpretation

Chocolate Agar Base

Neisseria gonorrhoeae is a gram-negative bacteria and the causative agent of gonorrhea, however it is also occasionally found in the throat. The cultivation medium for gonococci should ideally be a rich nutrients base with blood, either partially lysed or completely lysed. The diagnosis and control of gonorrhea have been greatly facilitated by improved laboratory methods for detecting, isolating and studying N. gonorrhoeae. Chocolate Agar Base, with the addition of supplements, gives excellent growth of the gonococcus without overgrowth by contaminating organisms. G.C. Agar (M434) can also be used in place of Chocolate Agar Base, which gives slightly better results than Chocolate Agar (1). The diagnosis and control of gonorrhea have been greatly facilitated by improved laboratory methods for detecting, isolating and studying N. gonorrhoea. Interest in the cultural procedure for the diagnosis of gonococcal infection was stimulated by Ruys and Jens, Mcleod and co-workers (2), Thompson (3), Leahy and Carpenter (4), Carpenter, Leahy and Wilson (5) and Carpenter (6), who clearly demonstrated the superiority of this method over the microscopic technique. Chocolate Agar Base with addition of supplement not only supports the growth of the gonococcus in pure culture but also permits its development from the mixed flora encountered in chronic gonococcal infections. Carpenter (7) reported that this medium and Haemoglobin (FD022) is useful for cultural detection of the Gonococcus.

MacConkey Agar

MacConkey Agar is the earliest selective and differential medium for cultivation of enteric microorganisms from a variety of clinical specimens (8,9). Subsequently MacConkey Agar and Broth have been recommended for use in microbiological examination of foodstuffs (10) and for direct plating / inoculation of water samples for coliform counts (11). These media are also accepted by the Standard Methods for the Examination of Milk and Dairy Products (12) and pharmaceutical preparations (13). Original medium contains protein, bile salts, sodium chloride and two dyes. The selective action of this medium is attributed to bile salts, which are inhibitory to most species of gram-positive bacteria. MacConkey Agar w/o CV, NaCl and W/ 0.5% Sodium taurocholate is a modification of the original formulation with the exclusion of crystal violet and inclusion of sodium taurocholate instead of bile salts. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. Lactose fermenting strains grow as red or pink and may be surrounded by a zone of acid precipitated bile. The red colour is due to production of acid from lactose, absorption of neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8. Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent and typically do not alter appearance of the medium. Yersinia enterocolitica may appear as small, non-lactose fermenting colonies after incubation at room temperature.