MMI 133: Session 7 - Fungi and Infection Control Notes

MMI 133: Session 7 - Fungi and Infection Control

Objectives Part 1: Fungi

Differentiate fungi from bacteria, viruses, and parasites.
Identify fungal structures that can be used as targets for anti-fungal drugs.
Describe the characteristics of "dimorphic fungi."
Define three types of fungal infections (superficial, cutaneous, and systemic) and name common dermatophytic diseases.
Define opportunistic fungal infections and provide examples.

General Characteristics of Fungi

Importance:
- Significant as nosocomial (hospital-acquired) and opportunistic infections, especially in immunocompromised individuals.
- Few are primary pathogens, capable of causing disease in healthy individuals.
- Many beneficial functions in nature, including decomposition of plant matter and as edible mushrooms.
- Used as leavening agents for bread and for fermentation in beer, wine, etc.
Cell Walls of Fungi:
- Composed of Chitin, which stains with calcium-binding agents like Calcofluor white, useful for rapid identification in clinical specimens.
Cell Membrane:
- Contains Ergosterol, which serves a similar function to cholesterol in human cell membranes.
- Ergosterol is a primary target for many antifungal drugs because it is unique to fungi and absent in human cells.
  - Amphotericin B: Most widely used antifungal that affects ergosterol.
  - Azoles: A class of drugs that inhibit ergosterol synthesis.
  - Nystatin: Another polyene antifungal that targets ergosterol.

Types of Fungi

There are three main types of fungi to remember:

Moulds
Yeasts
Thermally Dimorphic Fungi

A. Moulds

The body of a fungus is called a "thallus," which consists of masses of hyphae (e.g., the white powdery layer on moldy fruit).

B. Yeasts

Morphology: Oval or spherical in shape.
Reproduction: Primarily reproduce by budding.
Pseudohyphae: If buds fail to detach, they can form elongated chains called pseudohyphae.
Candida albicans:
- May attach to epithelial cells as a yeast form.
- Invades deeper tissues by means of pseudohyphae, which are crucial for its invasive capabilities.
Saccharomyces (nonpathogenic):
- A common yeast used for ethanol production (wine) and carbon dioxide for leavening bread dough.
Growth Conditions:
- Yeasts are capable of facultative anaerobic growth, meaning they can grow with or without oxygen.
- Most moulds, in contrast, are aerobic only.
- This difference is important for understanding their invasiveness in various environments within the body.
Cryptococcus neoformans:
- Produces a polysaccharide capsule, which is a key virulence factor.
- Often found in C.S.F. (cerebrospinal fluid) in infections, particularly in immunocompromised individuals.

C. Thermally Dimorphic Fungi

Possess two forms of growth: they can grow as a mould or a yeast.
Dimorphism is temperature-dependent:
- Pathogenic species grow as moulds at lower temperatures (e.g., $20^ ext{o} ext{C}$ in the environment).
- They grow as yeasts at body temperatures (e.g., $37^ ext{o} ext{C}$ within a host).
Examples of Dimorphic Fungi:
- Histoplasma capsulatum
- Blastomyces dermatidis
- Coccidioides immitis
Transmission: Dimorphic fungi are not infectious from one person to another. The mould form is spread by spores in the air, often exhibiting characteristic geographical distribution.
Sporothrix (causes subcutaneous infection):
- Also dimorphic and commonly known as "Rose Gardener's Disease."
- Fungal spores are introduced into a wound after trauma (e.g., from thorns).
- Lymphangitis (inflammation of the walls of lymphatic vessels) is common, usually affecting an arm or leg, and causes more ulcers as the disease progresses.

Fungal Reproduction and Nutrition

Reproduction: Fungi form "spores" called conidia (these are not true endospores like bacteria).
- Conidia are important for microscopic identification and for reproduction.
- They are very different from bacterial endospores in their formation and function.
Nutrition:
- Fungi are less nutritionally demanding than bacteria.
- They often grow better than bacteria in extreme conditions.
- Most fungi are aerobic and grow best at temperatures between $25^ ext{o} ext{C}$ and $30^ ext{o} ext{C}$ .

Fungal Diseases (Mycoses)

All fungal diseases are collectively called mycoses (plural) or mycosis (singular).

A. Types of Mycoses (from less severe to more severe):

Superficial Mycoses:
- Fungal infections along the surface of hair shafts and the outer layer of skin.
- Typically mild infections.
- Examples: Black and white piedra (affecting scalp hairs), Malassezia furfur (a common yeast cause).
Cutaneous Mycoses:
- Fungal infections of the deeper layers of the epidermis.
- Examples: Athlete's foot or ringworm.
- Causative Agents (Dermatophytes): These are three different types of moulds that degrade and use keratin (a protein found in skin, hair, and nails) as a carbon source (food).
- Clinical Diseases: Collectively called "tineas."
  - Tinea capitis: Involves the head.
  - Tinea pedis: Involves the feet (athlete's foot).
  - Tinea cruris: Involves the groin (jock itch).
  - Tinea corporis: Involves the torso (ringworm).
  - Tinea barbae: Involves the beard.
Systemic Mycoses:
- Infections that occur in deep tissues (organs).
- Causative Agents:
  - Primary pathogens: Such as the dimorphic fungi (Histoplasma, Blastomyces, Coccidioides).
  - Opportunistic pathogens: Such as Candida albicans and Aspergillus species.

Opportunistic Fungal Pathogens

Fungi are a common cause of infections in immunocompromised individuals.
Aspergillosis:
- Caused by Aspergillus species.
- Infects people with debilitating lung diseases (e.g., cystic fibrosis, chronic obstructive pulmonary disease) and those who are immunocompromised.
Candidiasis:
- Caused by Candida albicans, which is normally found as flora in the gut and mouth.
- Can cause vulvo-vaginal candidiasis (yeast infection) and thrush (oral candidiasis, common in newborns, immunocompromised individuals, and those treated with broad-spectrum antibiotics, as antibiotics disrupt normal bacterial flora allowing Candida to overgrow).

MMI 133: Session 7 - Infection Control

Objectives Part 2: Infection Control

Define common terms used to describe disinfection, sterilization, pasteurization, sanitization, and antisepsis.
Identify uses for positive and negative pressure isolation rooms.
Describe the process of sterilization and control of the process using the endospore test.
Identify chemicals that can be used for sterilization.
Explain why alcohol-based sanitizers are not appropriate for naked viruses and endospore-containing organisms.

Sterilization and Disinfection: Key Definitions

| Term | Definition |
| :----------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| |
| Antiseptic | A disinfectant used on living tissue, such as skin. |
| Aseptic Technique | The use of methods to exclude microorganisms from a specific area or during a procedure. |
| Bactericidal | An agent that kills bacteria. |
| Bacteriostatic | An agent that inhibits the growth of bacteria but does not necessarily kill them. |
| Disinfectant | A chemical used to destroy many microorganisms and viruses on inanimate surfaces (not typically used on living tissue). |
| Fungicide | An agent that kills fungi. |
| Pasteurization | A brief heat treatment used to significantly reduce the number of organisms and to kill pathogenic organisms (e.g., in milk or juice); it does not achieve complete sterilization. |
| Sanitization | Reduction of the number of organisms to a level that meets public health standards, making items safe for handling. This term is generally NOT USED in a hospital setting for critical items. |
| Sterilization | The complete killing of all forms of microorganisms, including highly resistant bacterial spores, and "inactivating" viruses. This is the highest level of microbial control. |
| Viricide | An agent that inactivates viruses. |

Environmental Control: Ventilation

Ultra-clean air: Achieved through High-Efficiency Particulate Air (HEPA) filters, which remove airborne particles, including microorganisms.
Building standards: Ensure correct airflows and pressure differentials in hospital settings.
Isolation rooms with "air locks" are crucial for preventing the spread of infection.
- Positive Pressure Rooms: Air from the corridor cannot get into the room. Used for immunocompromised patients (e.g., transplant recipients, severe burn victims) to protect them from environmental pathogens.
- Negative Pressure Rooms: Air from the room cannot get into the corridor. Used for patients with airborne infectious diseases (e.g., active tuberculosis, measles, varicella) to contain pathogens within the room and prevent their spread to other areas.

Physical Means of Controlling Microbial Growth

Heat:
- Hot Air (Dry Heat Sterilization): Used for glassware or powders. Requires higher temperatures and longer times, typically $160-180^ ext{o} ext{C}$ for $1-2$ hours.
- Autoclaving (Moist Heat Sterilization): Considered the most effective and widely used method for sterilization (except for heat-sensitive materials). It uses steam under pressure.
  - Standard conditions: $121^ ext{o} ext{C}$ , $15 ext{ psi}$ (pounds per square inch) pressure, for $15$ minutes. These conditions are sufficient to kill Clostridium botulinum spores, which is an industry standard for food sterilization.
Filtration: Physically removes microorganisms from liquids or air, often used for heat-sensitive fluids.
Cold: Inhibits microbial growth (bacteriostatic) but generally does not kill microorganisms.
High Pressure: Can kill some microorganisms by denaturing proteins and altering cell structures.
Desiccation (Drying): Inhibits growth by removing water, but many microbes can survive in a dormant state.
Osmotic Pressure: High concentrations of salt or sugar can inhibit microbial growth by drawing water out of cells.
Radiation:
- Ionizing Radiation (e.g., gamma rays): Can achieve sterilization by damaging DNA, used for heat-sensitive medical devices.
- Non-ionizing Radiation (e.g., UV light): Damages DNA and is used for surface disinfection, but has poor penetrating power.

Most important consideration for the efficiency of sterilization is whether the object to be sterilized is free of organic matter (e.g., blood, fecal material, tissue). Organic matter can shield microbes from the sterilizing agent.

"Spore Test" for Autoclave Effectiveness

This test is based on determining the ability of the autoclaving procedure to totally inactivate bacterial spores.
An ampule containing nutrient media and spores of a non-pathogenic bacterium, typically Bacillus stearothermophilus (a thermophilic spore-former), is placed inside the autoclave with the items to be sterilized.
After the autoclaving cycle, the ampule is incubated.
Indicator: The nutrient medium often contains a pH indicator.
- Positive test (not sterile): If the autoclave failed, the spores will germinate and grow, metabolizing nutrients and producing acidic or alkaline compounds, leading to a change in the pH of the medium, indicated by a color change.
- Negative test (sterile): If the autoclave successfully killed all spores, there will be no growth, and the medium's color will remain unchanged.

Chemical Means of Controlling Microbial Growth

Only a few chemical agents can achieve true sterilization.

Hand Soap: Primarily surface-active agents that aid in mechanical removal of microbes through washing.
Phenols: Good disinfectants, effective even in the presence of organic material (e.g., Lysol®).
Bisphenols: Antiseptics (e.g., Phisohex®).
Biguanides: Antiseptics (e.g., Chlorhexidine®).
Quaternary Ammonium Compounds ("Quats"): Disinfectants (e.g., Cepacol®).
Alcohols (Isopropyl/Ethanol):
- Microbicidal Action: Kill bacteria and fungi by protein denaturation and disruption of lipid membranes.
- Limitations: Do not kill endospores or non-enveloped (naked) viruses.
- Uses: Commonly used for skin "degerming" (reducing microbial count).
- Caution for Wound Treatment: Not ideal for treating open wounds, as they cause coagulation of proteins, creating a protective "scab" under which bacteria can continue to grow.
- Optimal Concentration: Ethanol is most effective at approximately 70 ext{%}, as the denaturation process requires the presence of water.
- Isopropanol (rubbing alcohol): Generally considered better than ethanol because it evaporates less quickly, allowing for longer contact time.
Heavy Metals: Silver and mercury compounds have antimicrobial properties.
Halogens (Chlorine, Iodine):
- Iodine: Available as a tincture (solution in alcohol) or as an iodophor (a combination of iodine and an organic molecule that slowly releases iodine, e.g., Betadine®).
- Chlorine: A strong oxidizing agent (e.g., Chlorox® bleach). Chlorine dioxide is used for water decontamination by backpackers, offering good effect without residual taste.
- Sterilization Potential: Halogens can sterilize if an appropriate amount of active chemical is used and prolonged exposure time is maintained. They are among the most used and useful disinfectants in everyday healthcare situations.
Aldehydes: Can sterilize (e.g., glutaraldehyde or Cidex®).
Ethylene Oxide: A gas that sterilizes by alkylating proteins and nucleic acids, often used for heat-sensitive items.
Peroxygens: Strong oxidizers that sterilize (e.g., hydrogen peroxide, peracetic acid).

Relative Resistance of Microorganisms to Chemical Agents

(From Most Susceptible to Least Susceptible)

Enveloped Viruses (Most susceptible)
Gram-positive Bacteria
Naked Viruses (More resistant than enveloped viruses)
Fungi
Gram-negative Bacteria
Vegetative Protozoa
Cysts of Protozoa
Mycobacteria (Due to their waxy cell wall, making them highly resistant)
Endospores (Highly resistant resting structures of bacteria)
Prions (Least susceptible, extraordinary resistance to sterilization methods)

Summary Part 2

Sterilization: The most rigorous process, specifically aims for the inactivation of bacterial spores.
Antisepsis: Refers to microbial control on living skin.
Disinfection: Refers to microbial control on inanimate surfaces.
Physical vs. Chemical Disinfection: Both are critical, but their application depends on the item to be treated and the level of microbial control required.
Considerations: Not all methods are appropriate for inactivating highly resistant organisms like mycobacteria and bacterial spores. The chosen method must align with the target microorganisms and desired outcome. Sterilants such as aldehydes, ethylene oxide, and sometimes halogens are needed for resistant organisms.