Adaptations of Candida albicans for growth in the mammalian intestinal tract

Abstract

Although the fungus Candida albicans is a commensal colonizer of humans, the organism is also an important opportunistic pathogen. Most infections caused by C. albicans arise from organisms that were previously colonizing the host as commensals, and therefore successful establishment of colonization is a prerequisite for pathogenicity. To elucidate fungal activities that promote colonization, an analysis of the transcription profile of C. albicans cells recovered from the intestinal tracts of mice was performed. The results showed that within the C. albicans colonizing population, cells expressed genes characteristic of the laboratory-grown exponential phase and genes characteristic of post-exponential-phase cells. Thus, gene expression both promoted the ability to grow rapidly (a characteristic of exponential-phase cells) and enhanced the ability to resist stresses (a characteristic of post-exponential-phase cells). Similarities in gene expression in commensal colonizing cells and cells invading host tissue during disease were found, showing that C. albicans cells adopt a particular cell surface when growing within a host in both situations. In addition, transcription factors Cph2p and Tec1p were shown to regulate C. albicans gene expression during intestinal colonization.

Fig. 1.

Expression of growth-phase-regulated genes during growth in the cecum. Relative expression of the genes described in Table 1 is shown. A white bar indicates expression in C. albicans cells grown in YPD for 5 h (exponential phase) relative to expression in cells grown in YPD for 3 days (post-exponential phase). A black bar indicates expression in C. albicans cells recovered from the cecum relative to expression in cells grown in YPD for 3 days. Genes 1 to 27 are expressed more highly in exponential phase than in post-exponential phase (white bar > 1.5). Genes 28 to 42 are expressed more highly in post-exponential phase (white bar < 0.67).

Fig. 2.

Genes upregulated during growth in the cecum and in laboratory conditions. (A) Expected number of genes that would be upregulated during both growth in the cecum and growth in another condition, based on chance (gray bar). The value was calculated from the frequencies of upregulated genes that were observed in each data set. The black bar shows the number of genes that were observed to be upregulated in both conditions. E, expected; O, observed. **, P < 10⁻³⁰; *, P < 10⁻¹⁰. (B) Percentage of genes in different classes. The white bar with thick lines indicates the percentage of hyphal genes (upregulated during hyphal growth in either the study of Kadosh and Johnson [25] or the study of Goyard et al. [19]). Genes that were not hyphal genes and were upregulated in at least one stress condition (oxidative stress [any of three time points] [15], osmotic shock [three time points] [15], temperature shift [three time points] [15], 3-AT [62], or starvation [carbon or nitrogen] [33]) were defined as stress genes (gray section). The percentage of genes that were neither hyphal nor stress genes and were upregulated in one of the other conditions is shown in the white section. The percentage of genes that were upregulated only in the indicated condition is shown in the black section.

Fig. 3.

Gene expression in C. albicans cells lacking Cph2p or Tec1p. RNA was prepared from C. albicans cells grown in the murine cecum. Expression of the genes indicated at top of graph was measured by quantitative real time RT-PCR and normalized using the expression of ACT1. Expression is shown relative to expression in WT laboratory-grown exponential-phase cells. Each symbol represents the average of triplicate measurements from a different RNA sample. Strains were as follows: black circles, WT; open circles, cph2-null mutant; gray circles, cph2/CPH2⁺ reconstituted mutant; black squares, tec1-null mutant. Black bar indicates mean.

Fig. 4.

Lack of Cph2p alters murine intestinal colonization. WT (DAY185 or SN100), cph2 deletion mutant, cph2UAU mutant, CPH2 reconstituted null mutant, or tec1UAU mutant were orally inoculated by gavage into Swiss Webster mice. At various days postinoculation, the amounts of C. albicans in fecal pellets were measured. The amounts of C. albicans in organs of the intestinal tract were measured after sacrifice. (A) CFU per gram of fecal pellet. Mice were sampled repeatedly, and each symbol represents a sample from a different mouse. Black triangles, WT C. albicans; black diamonds, cph2 mutant strain; gray diamonds, CPH2 reconstituted strain; bars, geometric means. (B) CFU per gram of organ or organ contents from mice sacrificed on day 21 postinoculation. Each symbol represents a sample from a different mouse. For the stomach and cecum, the colonization in the contents is shown. For the ileum, the contents plus wall were homogenized. Organs are indicated at the top of the graph. Black triangles, WT C. albicans; black diamonds, cph2 mutant strain; open diamonds indicate CFU/g below the limit of detection; gray diamonds, CPH2 reconstituted strain; bars, geometric means. (C) CFU per gram of fecal pellet. Mice were sampled repeatedly, and each symbol represents a sample from a different mouse. Black triangles, WT C. albicans; black diamonds, tec1UAU mutant strain; bars, geometric means.