In Vitro Acid Resistance of Pathogenic Candida Species in Simulated Gastric Fluid

Abstract

Background and aims: Although species in the fungal genus Candida are often commensal residents of the gastrointestinal (GI) tract, they can also cause high-mortality systemic candidiasis. Most pathogenic Candida species are dimorphic fungi that exist predominantly in filamentous forms in the invading tissues. Candida albicans is the most prominent pathogen among Candida species, but nonalbicans Candida species have also emerged as important pathogens. The stomach is the most acidic niche in the GI tract and is maintained at pH 1-2 in healthy individuals. The aim of the present study was to determine whether Candida species can survive in gastric fluid and to observe their morphology under varied pH conditions.

Methods: We investigated the in vitro survival of the pathogenic Candida species C. albicans, Candida glabrata, Candida parapsilosis, and Candida tropicalis in simulated gastric fluid.

Results: We first described that a portion of the 4 Candida species can survive under highly acidic conditions. Moreover, dimorphic Candida species, namely, C. albicans, C. parapsilosis, and C. tropicalis, exhibited yeast-hyphal transition in simulated gastric fluid with elevated pH. Pathogenic filamentous cells had lower acid resistance than yeast cells.

Conclusion: These findings may illuminate the migration to the lower GI tract by commensal fungi of the oral cavity.

Keywords: Artificial Gastric Juice; Candida Albicans; Candida Glabrata; Candida Parapsilosis; Candida Tropicalis.

Graphical abstract

Figure 1

Preincubated cells of 4 Candida species, C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis, were challenged in simulated gastric fluid (SGF) with an initial pH between 1.0 and 5.0 at 37 °C for 4 hours. Survival of the cells was assessed by counting the colonies. Data were presented as mean ± standard deviation (SD). The data in all panels are from 3 independent experiments performed in sextuplicate.

Figure 2

Comparison of Candida growth in simulated gastric fluid (SGF) adjusted to pH 1 at 37 °C and yeast extract-peptone-dextrose (YPD) broth adjusted to pH 1 at 30 °C for 4 hours. Survival of the cells was assessed by counting the colonies produced subsequently. Welch's t test was used to assess the statistical significance of differences between groups. Three independent experiments were performed in sextuplicate.

Figure 3

Growth forms of dimorphic Candida species cultured in SGF pH 1, 3, and 5 at 37 °C for 4 hours. (A) The growth form was recorded using a camera attached to a microscope. Green arrow indicates yeast cell and orange arrow indicates filamentous cell. (B)The numbers of yeast and filamentous cells were counted in more than 15 fields of view in each group, and the percentages of filamentous cells are indicated. Data were presented as mean ± SD.

Figure 4

Survival rate was compared between filamentous and yeast cells. Survival rate of C. albicans (A), C. parapsilosis (B), and C. tropicalis (C) in simulated gastric fluid (SGF) with an initial pH of 1.0. (D) The morphology was recorded using a camera attached to a microscope. Green arrow indicates yeast cell and orange arrow indicates filamentous cell. Welch's t-test was used for assessing statistical significance between groups. Three independent experiments were performed in sextuplicate.