Taxonomy and Morphological Properties
The genus Lactobacillus belongs to the family Lactobacillaceae and includes about a hundred different species of microorganisms. In microbiological classification, they are grouped with regular Gram-positive rods (along with the genus Listeria).
Despite belonging to the regular rod-shaped bacteria, lactobacilli are morphologically pleomorphic—their appearance under a microscope can be quite varied. In smears, they typically form short chains. On Gram staining, the bacteria consistently yield a Gram-positive (G+) result. An important characteristic of the genus is that its members do not form spores, and the vast majority of species are completely non-motile.
Physiology and Cultivation Features
Based on their type of respiration, lactobacilli are classified as aerotolerant anaerobes. This means that optimal growth requires oxygen-free conditions, but they are capable of surviving and maintaining viability even in the presence of oxygen.
In laboratory settings, these bacteria prove to be quite fastidious microorganisms. Their successful cultivation requires complex nutrient media necessarily enriched with amino acids. When grown on blood agar, colonies form a zone of greening hemolysis (α-hemolysis). A distinctive feature of lactobacillus metabolism is their extremely high saccharolytic activity: microorganisms actively ferment various carbohydrates with intensive acid production.
Role in the Human Body and Ecology
Members of the genus Lactobacillus are widespread in nature. In the human body, they are an integral part of the normal microflora of the gastrointestinal tract and oral cavity, and act as the dominant flora in the vagina.
The main function of lactobacilli is host defense. During their metabolic activity, they produce significant amounts of lactic acid, which helps maintain a stable acidic pH level on mucous membranes (within 3.8–4.5). Such an acidic environment effectively suppresses the proliferation of opportunistic pathogens, including:
- Mobiluncus
- Prevotella
- Gardnerella
- Mycoplasma
- Ureaplasma
When the population of lactobacilli decreases, dysbiosis develops. The loss of the protective acid barrier leads to the active proliferation of pathogens, clinically manifested as bacterial vaginosis and various vaginitises.
Clinical Significance and Resistance
In general, lactobacilli are characterized by extremely low pathogenicity. Infectious diseases caused by these bacteria are rare and develop predominantly in patients with severe immunodeficiency. In such cases, serious conditions may occur: bacteremia, endocarditis, intra-abdominal abscesses, as well as meningitis and conjunctivitis.
In the external environment, the bacteria are fragile—they rapidly die in open air and demonstrate high sensitivity to standard antiseptics and disinfectants. However, when treating infections, it is important to consider antibiotic resistance: microorganisms possess intrinsic resistance to vancomycin and all cephalosporin drugs. The drugs of choice for treating lactobacillus infections are traditionally penicillins, prescribed in combination with aminoglycosides.