Kanamycin-vancomycin laked blood agar (KVLB) is primarily used for the presumptive identification of which organisms?

• A. Clostridium perfringens
• B. Bacteroides fragilis group
• C. Bifidobacterium dentium
• D. Peptostreptococcus anaerobius

Answer: (B)

KVLB agar relies on selective inhibition to let a specific anaerobe stand out. The combination of kanamycin and vancomycin suppresses most Gram-negative and Gram-positive flora, respectively, so only certain anaerobes can grow. The Bacteroides fragilis group not only survives this antibiotic mix, but on laked blood agar they produce a distinctive dark to black pigment, which serves as a quick presumptive indicator of their presence.

So, organisms like Clostridium perfringens (a Gram-positive anaerobe) are inhibited by vancomycin and won't show the characteristic pigment. Bifidobacterium dentium and Peptostreptococcus anaerobius are also inhibited by the antibiotics and do not form the pigmented colonies seen with the B. fragilis group. The black-pigmented colonies on KVLB are the hallmark pointing toward the Bacteroides fragilis group.