Enhancement of Colonial Growth of Brucellae on Beef Liver Agar.

Beef liver agar (Stafseth, Mich. State Univ. Agr. Expt. Sta. Tech. Bull. No. 49, part 2, 1920) inoculated with large numbers of brucellae is one of the most satisfactory media for obtaining rapid and abundant surface growth. Certain aerobic strains of this genus fail to grow on liver agar plates when the surfaces are inoculated with 103 or fewer live cells. Attempts (Polding, Med. Res. Council Spec. Rept. Ser. 206, 1946) to ascertain and to remove the factor in liver agar that inhibits the colonial growth of small numbers of brucellae were not successful. The failure of small numbers of brucellae to grow on liver agar may be due to its high (+0.3 to 0.4) Eh potential (Tuttle and Huddleson, J. Infect. Diseases 54:1, 1934). In a recent study of strains of brucellae which failed to grow aerobically in colony form on liver agar plates, I found that all, except those of one group, grew rapidly when the inoculated plates (less than 103 cells per plate) were incubated in sealed glass jars containing 5% (v/v) CO2. The same strains also grew aerobically in colony form on liver agar when the medium contained 0.02 to 0.05% Na2S205 (Huddleson, Am. J. Vet. Res. 16:264, 1955). The number of colonies that appeared on the agar surfaces paralleled those on control plates