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 among regular-shaped Gram-positive rods (along with the genus Listeria).
Despite belonging to regular-shaped bacteria, lactobacilli are morphologically pleomorphic—their appearance under a microscope can be quite diverse. In smears, they typically form short chains. Upon 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 immobile.
Physiology and cultivation features
According to their respiration type, lactobacilli are classified as aerotolerant anaerobes. This means they require oxygen-free conditions for optimal growth, but are capable of surviving and maintaining viability even in the presence of oxygen.
In laboratory conditions, 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 distinct feature of lactobacillus metabolism is their extremely high saccharolytic activity: microorganisms actively ferment various carbohydrates with the intensive production of acids.
Role in the human body and ecology
Representatives 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 the protection of the macroorganism. During their metabolic activity, they produce significant amounts of lactic acid, which helps maintain a steadily acidic pH level on mucous membranes (within 3.8–4.5). Such an acidic environment effectively suppresses the proliferation of opportunistic microorganisms, including:
- Mobiluncus
- Prevotella
- Gardnerella
- Mycoplasma
- Ureaplasma
When the count of lactobacilli decreases, dysbiosis develops. The loss of the protective acidic barrier leads to the active proliferation of pathogens, clinically manifested as bacterial vaginosis and various vaginitis.
Clinical significance and resistance
Overall, 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 nosologies may occur: bacteremia, endocarditis, intra-abdominal abscesses, as well as meningitis and conjunctivitis.
In the external environment, the bacteria are unstable—they rapidly die in the open air and demonstrate high sensitivity to standard antiseptics and disinfectants. However, when treating infections, it is important to consider antibiotic resistance: microorganisms possess natural resistance to vancomycin and all cephalosporin drugs. The drugs of choice for treating lactobacillus infections are traditionally penicillins, which are prescribed in combination with aminoglycosides.
Briefly: answers to topical questions
Which macroorganism substrate do lactobacilli use to produce lactic acid in the vagina?
Lactobacilli produce lactic acid in the vagina by breaking down glycogen contained in the superficial cells of the vaginal epithelium. The production of lactic acid creates an acidic environment with a pH of 3.8–4.5, which is detrimental to many microorganisms and forms a natural barrier against pathogenic infection. A decrease in estrogen levels in postmenopausal women and prepubertal girls leads to a reduction in lactic acid production.
Remember the natural resistance of lactobacilli using the abbreviation VC: Vancomycin and Cefalosporins do not affect them.
Class and exam questions
Do lactobacilli form spores to survive in the external environment?
No, these bacteria are not capable of forming spores. As a result, they are quite vulnerable and quickly die when exposed to the air.
How do lactobacilli protect the vagina from infections?
They actively ferment carbohydrates with the release of lactic acid. This maintains a low pH level (3.8–4.5), which physiologically suppresses the growth of opportunistic bacteria.
Which antibacterial drugs are ineffective against lactobacilli?
The bacteria possess natural, genetically determined resistance to vancomycin and cephalosporin drugs.
Can lactobacilli cause a serious disease?
Yes, but this occurs extremely rarely and mostly in immunocompromised patients. In such patients, they can cause bacteremia, endocarditis, meningitis, or intra-abdominal abscesses.
What else to study on this topic
- Biochemical pathways of carbohydrate fermentation by lactobacilli
- Pathogenesis and laboratory diagnostics of bacterial vaginosis
- The role of lactobacilli in the oral and gastrointestinal microflora
- Mechanisms of natural antibiotic resistance in Gram-positive rods
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