Candida: the importance of microbiological analysis in the hospital environment

Currently, yeasts of the genus Candida are one of the major agents of nosocomial infection and represent a challenge for the survival of critically ill hospitalized patients. These yeasts are considered the main group of opportunistic pathogenic fungi, representing about 8-10% of the causes of nosocomial blood infections in ICUs.

FACTORS INFLUENCING CANDIDA INFECTION

The Candida is yeast which is part of the microbiota of the human and animal body, colonizing the skin and mucous membranes of the digestive and urinary tract, oral and vaginal tracts.

There are currently about two hundred yeasts of the genus Candida, however, just over 20 species are harmful to man.

The main factor in the development of infections by this yeast, occurs by the disruption of the parasite-host balance, triggered by changes in the tissue barrier, changes in the indigenous microbiota and by the compromise of the immune system. In severe illnesses associated with prolonged hospital stay, there is a greater occurrence in the disruption of this balance.

Surgical procedures of great complexity that lead to immunosuppression, loss of integrity of natural barriers through prolonged use of catheters, multiple invasive procedures, prolonged antibiotic therapy, AIDS, neoplasms and chemotherapy are some of the factors that contribute to the alarming increase in fungal infections in the units intensive care.

Infections caused by Candida spp

Infections caused by Candida yeasts, depending on the injury site, can be classified in two different ways: superficial or mucous candidiasis and deep or systemic candidiasis.

Superficial candidiasis is the most common infection among candidiasis, affecting the skin and mucous membranes, and is usually caused by C. albicans , which is the most common commensal species found in the mouth, vagina and gastrointestinal tract of healthy individuals.

Systemic candidiasis occurs preferentially in patients with compromised immunity, where the microorganism spreads through the blood, and can settle in vital organs such as the brain, heart and kidneys. So it is related to high mortality rates.

Species such as C. tropicalis, C. parapsilosis, C. krusei, and C. glabrata , are considered emerging, and have emerged as the cause of invasive nosocomial infections, especially in patients admitted to ICUs.

Candida tropicalis  on Sabouraud agar and Chromogenic candida agar (Source / Ref.: Microbiology in pictures)

In 2019 another species, Candida auris , attracted the attention of several entities around the world. It is a low virulence microorganism that causes symptoms only in hospitalized people. Most patients have sepsis, fever, hypotension and are resistant to antibiotics. Mortality from complications is lower than Candida albicans , but the great characteristic is that it presents fungal resistance and is difficult to diagnose, mainly by laboratories that are not used to it.

Culture of Biological Samples for Isolation of Fungi

In view of these conditions, monitoring with mycological examinations in biological samples of patients is recommended, such as blood, sputum, tips of intravascular catheters, peritoneal fluid and urine. Yeast positive cultures may only mean colonization, but they can lead to subsequent invasive disease.

A prospective study in surgical ICU patients, showed that 38% of 29 patients developed infection after colonization. Colonization can be demonstrated by analyzing 3 or more samples, collected from the same or different sites, from the same patient, on consecutive days.

The sample, after processing, can be used to isolate the etiological agent. Therefore, it should be sown in streak movements (zig-zag) on ​​the surface of solid culture media.

Culture mediums

The culture medium can be selected according to: type of sample and etiological agent, according to clinical suspicion.
To isolate fungi from any type of sample, non-selective media must be used, which allow the growth of pathogenic fungi and fast-growing molds (<7 days). These fungi, despite being environmental contaminants, can be agents of mycoses in susceptible patients, that is, they are potentially opportunistic.

The medium used in a mycology laboratory is Sabouraud Dextrose Agar (ASD), or Sabouraud agar. As a rule, an antibiotic is used to prevent the growth of bacteria that can harm the isolation of fungi. Chloramphenicol is the most suitable, as it resists autoclaving. It can be placed in both ASD and other culture media for fungi to enrich those media to isolate the fungus.

Selective means for pathogenic fungi, contains cycloheximide that partially or totally inhibits anemophilic fungi. These media are indicated for the cultivation of materials collected from lesions with suspected dermatophytosis, to increase sensitivity in the isolation of dermatophytes. However, it should be noted that this substance may inhibit the isolation of opportunistic fungi, such as Aspergillus sp, in addition to Histoplasma capsulatum in the yeast phase and certain pathogenic yeasts of the genera Candida and Cryptococcus .

There are presumptive means that indicate the presence of certain groups of fungi or certain genera, such as, agar containing phenolic compounds for Cryptococcus sp (agar containing niger seeds or Guizzotia abssinica ), or special agar for dermatophytes ( Dermatophyte Test Medium ). There are also presumptive means, by enzymatic and colorimetric reaction, of species of Candida spp (Candida Medium, ChromAgar, Biggy Agar, etc.). They are more expensive than ASD and, moreover, their greatest application is in the primary isolation of yeasts, from highly contaminated biological samples, such as: vaginal discharge, feces and urine. The identification, however, is done only, after morphological and physiological analysis.

Potato agar,
commercially available in dehydrated form, is recommended for isolation or subculture of dermatophytes to increase sporulation and facilitate the identification of the fungus genus and species. For dimorphic fungi, of slow growth (> 15 days), it is recommended to use enriched media such as Brain Heart Infusion Agar (BHI) to obtain, in less time, better developed cultures. 5 to 10% of sheep’s blood and antibiotics (preferably chloramphenicol or penicillin and streptomycin) can be added.

 

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