Microbiology, Immunity and Forensics flashcards

Topic 6: Microbiology, Immunity and Forensics

Why is aseptic technique used, and why are plates taped with short strips and inverted?

To keep unwanted microbes out of the culture and the culture away from you. Short tape strips let oxygen in, so anaerobes are not encouraged; inverting stops condensation falling on colonies.

Dilution plating: viable count formula

CFU per cm³ = (mean colonies ÷ volume plated in cm³) × dilution factor. Count plates with about 30 to 300 colonies. It counts living cells only.

Phases of a bacterial growth curve

Lag: cells adapt and make enzymes; little change in number. Exponential: numbers double at regular intervals. Stationary: division rate equals death rate. Death: death exceeds division as nutrients run out and toxic waste builds up.

Why does turbidity stay high in the death phase?

Dead cells still scatter light, so turbidity counts living and dead cells. Only a viable count (dilution plating) shows the fall in living cells.

Bacterium vs virus

Bacterium: prokaryotic cell; circular DNA and plasmids; peptidoglycan wall; 70S ribosomes; own metabolism; binary fission. Virus: acellular; DNA or RNA in a capsid, some with an envelope; no ribosomes or metabolism; reproduces only inside host cells.

Lytic cycle vs latency

Lytic: the virus replicates at once, assembles new particles and bursts the host cell. Latency: viral genetic material is in the host but makes no new particles (lambda phage DNA in the bacterial chromosome; HIV provirus). It can later switch to the lytic cycle.

How does HIV infect a T helper cell?

Attachment proteins bind to CD4. The envelope fuses with the cell surface membrane. Reverse transcriptase makes DNA from viral RNA, which is integrated as a provirus. The host makes viral proteins and RNA, and new viruses bud off.

How does Mycobacterium tuberculosis cause TB?

Inhaled bacteria are engulfed by alveolar macrophages but resist digestion. Tubercles wall off the infection, which is often latent. If immunity weakens it reactivates, destroying lung tissue.

Roles of T helper, T killer, plasma and memory cells

T helper: releases cytokines that activate B cells, T killer cells and phagocytes. T killer: destroys infected cells by direct contact (perforin). Plasma cell: secretes antibody. Memory cells: long-lived; give a fast secondary response.

Natural and artificial, active and passive immunity

Natural active: recovering from an infection. Artificial active: vaccination. Natural passive: antibodies across the placenta or in breast milk. Artificial passive: injected antibodies (antivenom, tetanus immunoglobulin).

How do pathogens evade the immune system? Give examples

Antigenic variation (influenza), so memory cells do not recognise it; destroying T helper cells (HIV); surviving inside macrophages (M. tuberculosis); latency; capsules that cover antigens.

Bacteriostatic vs bactericidal antibiotics

Bacteriostatic: stops growth and reproduction, for example tetracycline, which blocks bacterial protein synthesis. Bactericidal: kills bacteria, for example penicillin, which stops cell wall formation, causing lysis.

How do saprotrophs decompose dead matter?

They secrete enzymes (proteases, lipases, carbohydrases) onto the material; large molecules are hydrolysed to small soluble ones, which are absorbed. Some are respired, releasing CO₂ back to the air.

PCR: components and cycle

Template DNA, two primers, free nucleotides, Taq polymerase, buffer with Mg²⁺. Denature at about 95 °C; anneal primers at about 50–65 °C; extend at about 72 °C. Copies ≈ starting number × 2ⁿ.

How many PCR cycles turn 10 target molecules into more than one million copies?

10 × 2¹⁶ = 655 360 (too few). 10 × 2¹⁷ = 1 310 720. So 17 cycles.

Rigor mortis

Stiffening of muscles after death because no ATP is available, so myosin heads cannot detach from actin. Along with body temperature, decomposition and insects, it helps estimate time of death.