The shapes and sizes of the bacterial cells can vary greatly. They can be vimineous, stubbed, bent, bacilliform, clavate, club-shaped, etc. However, most Lactobacillus cells are fairly regular with no branching. The cells are square or obtuse at the ends when compared with other gram-positive nonsporulating bacilli. They produce no spores and no capsules and stain gram-positive.
Surface culture on a solid medium is best when performed in anaerobic or microaerophilic conditions. However, some species must be cultivated under anaerobic conditions. Some members of this genus can grow within the 15–45
°C range, in the presence of 5–10% CO2
, which promotes bacterial growth. As acid-producing bacteria, low pH Rogosa agar is the culture medium of choice for many strains of Lactobacillus
. The optimal pH for growth is 5.5–6.2.
The colonies are round, white or gray, transparent or nontransparent with a diameter from pinprick-sized to 2 mm on the agar surface. Smooth colonies are soft, raised, and lustrous, and the edge of the colony is neat. The surface of rough colonies is dry, flat, and lackluster, and the edge is not neat. The bacteria normally do not produce pigment.
Lactobacilli can ferment glucose to produce acid, and are negative for catalase, urease, and cytochrome enzyme. They do not produce benzpyrole, cannot reduce nitrate, cannot hydrolyze gelatin, and cannot produce H2
S. Sugar fermentation test and arginine hydrolysis test can help identify the genus of bacteria.
The bacteria can promote the development of tooth decay, as its detection is significantly increased in deep caries material.