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Microbial cultures and their clinical importance

By 1878, cultures were already being carried out in liquid medium, but these were not suitable for growth, since it was difficult to obtain pure cultures. It was not until 1882 that the Hessen spouses introduced agar-agar (polysaccharide extracted from red algae) as a solidifying agent.

Knowledge of how to cultivate and maintain microorganisms allows a better study of the causal agents of infections.

Bacterial growth is the increase in the number of bacteria in a culture medium and they need multiple physical and nutritional factors for their growth.

The physical factors are mainly a temperature between 35 – 37 degrees and optimum pH between 5.5 and 8.5. Other no less important factors are the osmotic pressure and adequate humidity conditions as well as oxygen levels.

Nutritional factors are very important for bacterial growth, in fact there are some germs that require culture media enriched with elements such as Carbon, Nitrogen, Phosphorus, Sulphur, Potassium, Magnesium, Calcium, trace elements mainly Iron, Copper and Zinc and other organic factors that bacteria are unable to synthesize.

The process of bacterial identification begins with the adequate sowing of the samples obtained from the patient. This is a complex, laborious task and requires specialized personnel.

In specialized microbiology laboratories, the identification of microorganisms is also carried out by gas-liquid chromatography tests, direct immunofluorescence, ELISA and molecular DNA tests.

However, regardless of the identification technique used, the microbiological culture continues to be the Gold Standard and until now is the method that allows knowing the antibiotic susceptibility through the antibiogram.

Sometimes we are faced with critically ill patients, where early antibiotic initiation is necessary to change the final prognosis. According to current sepsis guidelines, antibiotics should be started within the first hour of patient arrival. Microbiological samples must be taken before starting treatment, to adequately identify the bacteria and also to know the susceptibility profile.

Of note, once antibiotics have been started, the microbiota changes and etiologic agents are affected, leading to potentially misleading culture results and therapeutic failure.

The best option for the treatment of infections is taken based on the antibiogram, which we obtain from the bacterial culture. With the antibiogram we can determine the phenotypes of bacterial resistance and based on this make an appropriate prescription of antibiotics.