Yeasts

Yeasts

  • Some are normal flora (ex: Candida albicans)

    • Endogenous infection

  • Helpful to make beer and bread (Saccharomyces cerevisiae)

  • Opportunistic v. virulent pathogen those not part of normal flora causing disease

  • Yeasts are eukaryotic unicellular organisms that produce by budding

Clinically Important Yeasts

  • Candida

  • Cryptococcus - get from birds

  • Rhodotorula

  • Geotrichum

  • Pneumocystis

  • Saccharomyces

  • Note: Candida albicans and Cryptococcus neoformans are most common yeasts causing human disease

Laboratory ID

  • Yeasts are obtained from any specimen source

  • Proper ID relies heavily on biochemical ID leading nicely to commercially developed

  • No special precautions or procedures required such as in molds (universal precautions).

  • Can be in urine, body fluids, skin scrapings, tongue scrapings

Yeast Identification

  • Examine colony color, shape, texture, topography

  • Examine microscope morphology

    • Blastoconidia, Chlamydospores, Arthroconidia, etc.

  • Unstained slides - Add 1 drop of saline to CSF sediment or aspirates and coverslip

  • Stained slides - gram stain (all fungi are gram positive) - Wright’s or Giemsa stain for blood smears - PAS and GMS for tissue sections to identify fungi

  • Germ tube production

    • Candida albicans (+)

    • Proceed to further biochemical studies (-)

  • Note: Candida albicans is most commonly isolated yeast in the clinical setting. Most labs stop only further work - up if germ tubes are identified as they can be considered diagnostic for Candida albicans. May not always produce germ tubes however

  • Yeast or hardly and are not lysed when RBCs are

Candida

  • candida albicans is normal flora of the mouth, throat, vagina, lower GI, and most common cause of candidiasis

    • C. albicans though classified as a yeasts (unicellular eukaryotic fungi, by definition), they produce true hyphae (which is also not truly multicellular when they do they are associated with more severe disease

  • May be primary or secondary and can effect different body sites

    • Thrush (oral candidiasis) - occurs in newborns - low mouth pH and lack of normal flora or infected mother

  • Can also occur on older children/adults - immunosuppressed or Immunocompromised (HIV, chemo, long standing antibiotic use)

    • bronchopulmonary candidiasis

    • Cutaneous candidiasis (burn patients)

    • Nails (onychomycosis)

    • Endocarditis

    • UTIs - older people who are really sick

    • Vaginitis/ balanitis

    • CNS candidiasis, CSF

Candida albicans

  • Growth

    • Rapid - 3 day

  • Colony morphology

    • Cream colors, pasty, smooth

    • On BAP / chocolate - feet develop

  • On primary media

    • Yeast are round to oval

  • Germ tube test positive

Germ Tube Test

  • Take 0.5ml of rabbit, fetal calf or human serum and place into test tube

  • Take a small sample of the organisms in a question from the plate and place into test tube

    • Do not take too much in inoculum, too much impedes germ tube production

  • Incubate at 37 degrees for three hours

  • Take small drop, place on slide and coverslip and examine for germ tubes

Transmission of Candida

  • Community onset Candidiasis

    • Becoming more prevalent in IV drug abusers (heroin)

  • Nosocomial Candidiasis

    • Endogenous - intervenors catheters can get seeded with candida on the skin

    • Exogenous - horizontal transfer of candida from improper handwashing by health care workers (use of hand sanitizer)

Cryptococcus neoformans

  • Worldwide distribution

  • Not considered part of normal flora

    • Has been found in feces of normal humans, asymptomatic carriers of fungus

  • Typically inhaled - pulmonary infection

    • Not spread from person to person

    • Acquired typically through inhalation of spores, tied to pigeon droppings

  • Feces high in creatine good source of nitrogen for C. neoformans

    • Studies found many children under 2 in New York had antibodies against C. neoformans antigens

    • People inhale and commonly has latency period in immunocompetent

  • Immune system walls off spores in granulomas in the lung, which are held in check but may remain viable (capable of infecting when immunosuppressed)

  • Immunocompromised at risk

  • Typically observed as disseminated disease in immunocompromised people (AIDS, etc.)

    • 2/3rds of these patients experience meningitis

  • Rapids ID is necessary

Cryptococcus

  • Cryptococcus can be

    • Pulmonary

      • Cough, chest pain, SOB

    • CNS

      • Meningitis indicating hematogenous

      • See multiple pinpoint area of hemorrhage

  • Growth

    • Rapid-3 days

  • Capsule - best demonstrated with an India ink prep

  • Capsules are not phagocytized, it also interferes with complement

  • Causes apoptosis of CD4+ (helper levels)

Pneumocystis jiroveci

  • “yeast- like” fungus

  • Formerly Pneumocystis carinii

  • Initially though to be protozoan (cyst and toph. stages present)

    • Analysis of RNA and Mitochondria proved it to be a fungi and not a parasite

  • Found in nature; Airborne transmission

  • Immunocompromised, debilitated most at risk - leukemia, AIDS, malignancies

  • First opportunistic infection to appear in AIDS patients typically

  • Diffuse, interstitial plasma cell pneumonia

    • CT- display diffuse infiltrations with a “ground glass appearance”

  • Resembles Mycoplasma pneumoniae

  • Multiplies in lung alveoli - copious secretions produced - alveolar distention

  • Specimen - respiratory secretions, lung biopsy, brushings

  • Cysts

    • non-budding

    • Round, ovoid, collapsed crescent shaped

    • GMS stain: dark spots in shape of single or double commas or a set of parenthesis

    • looks like flattened footballs

  • Rapids tests exist

Rhodotorula spp.

  • Commonly a contaminant

  • Increasing in severe infection

    • Immunosuppressed with venous catheters

  • Growth

    • 4 days

  • Colony morphology

    • Pink to coral, can be orange to red

  • Microscopic morphology

    • Budding

    • Sometimes capsule is formed

Saccharomyces

  • Increasing as agent of infection

  • Growth

    • Rapids - 3 days

  • Colony morphology

    • Smooth, moist, white to cream colored

  • Microscopic morphology

    • Round to oval yeast

    • Multilateral budding observed

Geotrichum candidum

  • Produce arthroconidium

  • Normal flora of mouth, skin, GI

  • Must be differentiated from coccidiosis emits which is also produce arthroconidia

  • Has no sexual reproduction (imperfect), can be used to make cheese