Microbiology and Infection Quiz

Infection

• Infections are caused by microorganisms called pathogens that enter the body.
• Major types of pathogens include bacteria, viruses, fungi, and parasites.
• Transmission of infection involves a chain of events for one human to infect another.
• See Figure 8-1

Transmission of Infection

• Pathogens must live and reproduce in a reservoir (e.g., human for influenza, animal for rabies).
• Pathogens need a portal of exit and a mode of transmission from the reservoir to a susceptible victim (e.g., semen in sexual contact).

Direct vs. Indirect Transmission

• Direct transmission involves contact between the reservoir and the new host.
• Indirect transmission involves the reservoir depositing the pathogen on an inanimate object (fomite) that transmits it to a new host.
• Examples of direct contact include physical contact, fecal contamination, and airborne droplets.

Transmission of Infection (Continued)

• Pathogens gain entry to a victim through portals such as the nasal cavity or skin puncture.
• If the host is susceptible, the pathogen multiplies and overwhelms the host's immune system, leading to infection.
• The cycle repeats as the infected host becomes a reservoir.
• Factors affecting susceptibility include age, gender, genetics, immune system state, and stress level.

Colonization

• Colonization occurs when there's balance between a host and microorganism, controlling the location and total number of the pathogen in the host.
• Pathogens may not necessarily cause infectious diseases but can be transmitted to other hosts.

Control of Disease

• Breaking the transmission chain involves identifying vulnerabilities in the chain.
• Examples include rabies vaccinations to eliminate a reservoir and having patients with TB wear masks to block portal of exit.

The Host

• The human body has mechanical and biochemical barriers.
• Epithelial barriers prevent foreign material entry.
• The skin provides multiple protective layers, especially when dry (microorganisms prefer moist).
• Constant shedding of the epidermis and mucous membranes help with removal.
• High fat content of the skin inhibits bacterial and fungal growth.

Mucous Membrane Linings

• Mucous membrane linings separate the internal body, which is sterile, from the external environment.
• Respiratory tracts have mucous to trap and remove microorganisms.
• Chemical barriers such as skin, urine, and vaginal acidity inhibit bacterial growth.
• Stomach acid (pH 1-2) is inhospitable for many microorganisms.

Antimicrobial Chemicals

• Saliva, mucous, tears, and sweat contain antimicrobial chemicals, such as lysozyme, that destroy bacterial cell walls.

Lactoferrin & Sebaceous Secretions

• Lactoferrin in body fluids (saliva, tears, mucous) reduces free iron, essential for bacterial replication.
• Sebaceous gland secretions act as antifungals.

Immunoglobulins

• Immunoglobulins (IgA and IgG) prevent entry of bacteria and viruses through mucous membranes.

Infection (Continued)

• If body barriers are removed or compromised, infection is likely.
• Individuals with missing skin (burn victims) are at high infection risk.
• Urine flushes bacteria in healthy urinary tracts; catheters disrupt this.

Immunization

• Immunization is a cost-effective method to reduce susceptibility to pathogens.
• Immunization goals include giving immunity to the host by exposing them to altered (dead or weakened) pathogens.
• Immunization aims to decrease the number of susceptible hosts to limit transmission.

Microbial Adherence

• Microbial adherence is the microorganism's capacity to attach to and enter the host.
• Pathogens directly penetrate or adhere to hosts' tissue surfaces.
• Adhesion molecules on pathogen surfaces aid this.

Slime Layer

• Microbial development of a slime layer (typically mucopolysaccharide) aids adherence.
• Glycocalyx (slime layer) acts as an adhesive, sticking bacteria to host surfaces.
• Streptococcus mutans produces a slime layer for tooth enamel adherence, forming plaque.

Virulence

• Virulence is the ability of a microorganism to evade host defenses and cause disease.
• Higher virulence means a greater likelihood of causing harm to the host.
• Virulence and invasiveness factors help pathogens penetrate natural barriers and resist antimicrobial therapy and phagocytosis.

Bacterial Enzymes

• Bacterial enzymes (fibrinolysin, coagulase, hyaluronidase) aid in pathogen spread or invasion of tissues.
• These dissolve blood, collagen, or connective tissues.
• For example, fibrinolysin produced by streptococci dissolves coagulated plasma to spread microbes into host tissues.

Hyaluronidase

• Hyaluronidase, secreted by staphylococci, clostridia, and streptococci, helps pathogens spread by dissolving hyaluronic acid, a connective tissue component.

Encapsulation & Endospores

• Encapsulation prevents antibody recognition and phagocytosis.
• The bacterial slime layer prevents phagocytosis.
• Some microorganisms make endospores to withstand harsh environments (e.g., lack of moisture, heat).

Antimicrobial Resistance

• Microorganisms can mutate to adapt to changes in host environments and develop antimicrobial resistance.
• Insufficient antibiotic courses contribute to the evolution of antibiotic-resistant strains.

Viruses

• Viruses are small infective agents (20-300nm) composed of DNA/RNA with protein capsids.
• They replicate inside host cells, needing their energy.
• Some viruses form envelopes with host membrane and viral proteins.
• Viral infections are, in many cases, self-limiting, & few antiviral agents exist.

Viral Infection

• Viruses invade host cells to replicate.
• Viral adherence to cell membranes initiates entry and often causes phagocytosis.
• Inside, the virus' genetic material is exposed for replication within the host's cell
• Some viruses inject genetic material into host cell without engulfment.

Two Categories of Viruses

• DNA viruses (e.g., herpes simplex) first produce messenger RNA in the host cell nucleus.
• Host cell enzymes then replicate the viral DNA within the host cell.
• RNA viruses (e.g., influenza) replicate alongside host cell DNA.

Fungi

• Fungi are eukaryotic protists that can reproduce through division or genetic combination.
• Fungal infections are called mycotic infections or mycoses.
• Fungi colonize areas by adherence and proliferation.
• Fungi like Candida live naturally in the body as normal flora; overgrowth can occur when the immune system is compromised.

Patients on Antibiotics and Fungi

• Patients on antibiotics lose some normal flora, making them more susceptible to fungal overgrowth.
• Fungal infections are frequent in individuals with weakened immune or those with high nutrient environments.
• Topical fungal drugs can treat superficial infections.

Parasites

• Parasites (protozoa, roundworms, flatworms, arthropods) live in or on humans, relying on the host for survival.
• Symptoms depend on the parasite's location; for example, protozoan infections in the gut can cause cramping, abdominal pain, and diarrhea.
• Infections can be identified by microscopic examination of blood, feces, or tissue samples.

Types of Pathogens (Bacteria)

• Bacteria are single-celled organisms without internal organelles.
• Bacteria are grouped by cell wall shape and movement mechanisms: gliding, spirochetes, mycoplasmas, and rigid bacteria.

Free-Living Bacteria

• Rigid bacteria, including free-living bacteria, come in three basic shapes: cocci (round), bacilli (rod-shaped), and spiral.
• Free-living/rigid bacteria may be grouped in clusters, chains, pairs, or tetrads.

Gram Stain

• Gram stain reaction depends on the ability of bacteria to retain a basic dye after iodine fixation.
• Gram-positive organisms stain dark purple, gram-negatives appear pink.