Research algorithm and result interpretation
The work of a microbiologist with cultures is strictly regulated and includes several sequential stages. First, primary inoculation of clinical material is performed on a special set of nutrient media. Then, the character of growth is evaluated: the specialist gives a quantitative description, noting single colonies, moderate, or abundant growth. After this, a pure culture is isolated and the pathogen is definitively identified.
The key task is to determine the etiological significance of the discovered microbe. If obligate pathogens are detected, this unequivocally indicates the cause of the disease. The situation is more complex with opportunistic pathogenic bacteria (OPB), as in some loci (e.g., the external ear or ear canal) they are present as normal microflora. To confirm the role of OPBs in the development of infection, two criteria are used: the presence of a high concentration (titer) of the microbe and the reproducibility of isolating the same culture in a series of analyses (result reproducibility).
Microbiological principle of chemotherapy
The priority in treating infections is prescribing drugs based on proven microflora sensitivity, i.e., according to the results of bacteriological culture and antibiogram. However, in practice, drugs are often prescribed at the first signs of infection (e.g., fever), even before the pathogen is identified.
In such cases, empiric therapy is used. It should be based on a retrospective analysis of verified pathogens in a specific hospital and their antibiotic susceptibility data.
According to clinical use, antibiotics are divided into two groups:
- First-line (drugs of choice): prescribed first for empiric therapy before culture results are obtained.
- Reserve (second-line): used as a "second choice" if microorganisms are resistant to first-line drugs or if the patient is intolerant to them.
When choosing a drug, priority is given to narrow-spectrum agents. Broad-spectrum agents are justified in severe infections prior to precise pathogen identification.
Rules for rational combination
Combination therapy is indicated for severe infections of unknown etiology (using 2–3 drugs for maximum coverage), complicated forms (carbapenems — imipenem, meropenem are prescribed), and when multiple pathogens are involved, such as gonococci, Chlamydia, and anaerobes. When combining drugs, it is important to consider the nature of their effect.
Rational combinations (synergism):
- Aminoglycosides + Penicillins: classic bactericidal synergism. Beta-lactams disrupt cell wall synthesis, facilitating the penetration of aminoglycosides into the cell to ribosomes. Often used in severe pathologies such as endocarditis.
- The combination of antibiotics with sulfonamides, nitrofurans, and quinoxaline derivatives is widely used.
Irrational combinations (antagonism and toxicity):
- Tetracyclines + Penicillins: pharmacodynamic antagonism. Bactericidal penicillins act only on actively dividing cells, while bacteriostatic tetracyclines suppress division. Efficacy drops sharply.
- Macrolides + Lincosamides: competitive antagonism. Both groups bind to the 50S ribosomal subunit, competing for the target and promoting cross-resistance.
- Aminoglycosides + Polymyxins: dangerous summation of nephrotoxicity and neurotoxicity (up to the risk of neuromuscular blockade).
- It is forbidden to combine drugs of the same group (e.g., streptomycin, kanamycin, monomycin, gentamicin) due to a sharp increase in ototoxic and nephrotoxic effects.
Overcoming resistance and special cases
Bacterial resistance often has group specificity (cross-resistance): if a microbe is resistant to one drug, it will with high probability be resistant to its chemical analogues as well. It is impossible to completely prevent this process, but it can be slowed down.
Prevention measures include strict indications for prescription (antibiotics complement, but do not replace surgery), avoiding unjustified prophylaxis, rotating drugs every 10–15 days of therapy, and non-medical restrictions (ban on using antibiotics as animal growth promoters in veterinary medicine).
Special principles of tuberculosis therapy: Treatment of mycobacterial infections requires a special approach. The course of therapy is always very long — from 6 months to 1 year or more, which is necessary to achieve a stable effect and prevent relapses. To overcome mycobacterial resistance, an exclusively combined regimen is used: the simultaneous prescription of two or more drugs is a mandatory condition.
To remember the irrational combination of "Penicillins + Tetracyclines", imagine a construction site: penicillin destroys the wall under construction (acts on actively dividing cells), while tetracycline stops the construction itself (bacteriostatic). No cell division means penicillin has nothing to act on.
Class and exam questions
How to prove that an isolated opportunistic bacterium is the cause of the disease?
It is necessary to detect it in high concentration (high titer) and confirm result reproducibility by isolating the exact same culture in a series of repeated analyses.
In which cases are reserve antibiotics prescribed?
They are used as a second choice: if the pathogen proves resistant to the main drugs of choice or if the patient exhibits intolerance to the primary agents.
Why is it dangerous to combine aminoglycosides and polymyxins?
Both of these groups exhibit pronounced nephrotoxicity and neurotoxicity. When used together, toxic effects are summated, significantly increasing the risk of complications, including neuromuscular blockade.
What else to study on this topic
- Quantitative growth characteristics: single colonies, moderate and abundant growth
- Mechanisms of cross (group) bacterial resistance formation
- Features of empiric therapy based on hospital retrospective analysis
- Pharmacodynamic antagonism of bactericidal and bacteriostatic drugs
- Specifics of antibacterial therapy in mixed infections
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Bacteriological culture for microflora" and 4,383 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the "Additional Topics" section
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.