Microbial Death & Sterilisation – Key Concepts

Question–answer flashcards summarising definitions, structural differences, reasons for sterilisation, and major methods (filtration, heat, irradiation, chemical agents) with their advantages and limitations.

What is the definition of sterilisation?

The complete destruction or removal of all microorganisms, including endospores.

How does disinfection differ from sterilisation?

Disinfection removes or destroys harmful microorganisms but may not eliminate endospores.

What is sanitization?

Reducing microbial numbers to public-health safe levels rather than achieving total sterility.

What does the suffix “-cidal” (e.g., bactericidal) indicate?

It indicates agents or processes that lead to microbial cell death.

What does the suffix “-static” (e.g., bacteriostatic) indicate?

It indicates inhibition of microbial growth or reproduction without immediate cell death.

Define an antiseptic.

A chemical applied to living tissue to prevent infection by inhibiting or killing microbes.

What is meant by the broad term antimicrobial?

Any substance that kills or inhibits microorganisms, encompassing disinfectants, antiseptics, and antibiotics.

State one major structural feature of Gram-positive cell walls.

They contain a very thick layer of peptidoglycan with teichoic acids.

State two key structural features of Gram-negative cell walls.

A thin peptidoglycan layer plus an outer membrane containing lipopolysaccharide (LPS).

Why is LPS called an endotoxin?

Because it can trigger strong immune responses and fever (pyrogenic) when released.

What is the periplasmic space?

The area between the inner and outer membranes of Gram-negative bacteria.

Why are Gram-negative bacteria often more resistant to disinfectants?

The outer membrane acts as an additional permeability barrier.

Give one global public-health reason for sterilisation.

To reduce diarrhoeal diseases, a leading cause of death especially in low-income countries.

What is a healthcare-associated (nosocomial) infection?

An infection acquired in a hospital or clinical setting that increases mortality and readmission rates.

List the five practical questions to consider when choosing a sterilisation method.

1) How widely applicable? 2) Is it selective? 3) How certain is the outcome? 4) Will it damage the item? 5) Is it scalable/cost-effective?

What pore size is typically used in membrane filtration to remove bacteria?

About 0.22 µm.

Name one disadvantage of membrane filtration.

Standard filters often fail to retain all viruses.

For which two physical states of matter is membrane filtration mainly suitable?

Liquids and gases.

What temperature range (°C) is common for dry-heat sterilisation in hot-air ovens?

150–170 °C.

Why is dry-heat sterilisation generally slow?

Heat must penetrate layer by layer until the core reaches the target temperature.

Give one example of dry-heat sterilisation in the microbiology lab.

Flaming an inoculating loop in a Bunsen burner.

What device is commonly used for moist-heat sterilisation?

An autoclave.

How does an autoclave achieve sterilisation so quickly?

By using pressurised steam that transfers moist heat efficiently and denatures proteins irreversibly.

Why is moist heat unsuitable for many metallic objects?

Moisture can promote corrosion or rust on metal instruments.

Do cold temperatures sterilise? Explain briefly.

No; freezing halts growth but many microbes survive and can regrow after thawing.

What type of organisms require >100 °C to kill and are associated with hot springs?

Hyperthermophiles.

Differentiate ionising and non-ionising radiation in sterilisation.

Ionising (e.g., gamma) produces free radicals that break DNA; non-ionising (e.g., UV, IR, microwave) mainly damages by heat or DNA dimer formation.

Name two medical items often sterilised by gamma irradiation.

Disposable syringes and catheters.

Why is UV radiation limited to surface sterilisation?

It has poor penetration through solids and turbid liquids.

Which radiation type heats microbes internally by molecular vibration?

Infrared (IR) radiation.

Give one major disadvantage of ionising radiation facilities.

High cost and the need for stringent safety controls around radioactive sources.

What is formaldehyde gas used to sterilise?

Surgical/dental instruments and ventilation ducts.

Why is peracetic acid preferred over formaldehyde for some applications?

It is highly effective yet has a better safety profile (less carcinogenic).

State three mechanisms by which alcohol kills microbes.

Cell dehydration, membrane disruption, and protein denaturation.

Why is 70 % ethanol commonly used instead of 100 %?

Water content aids penetration and prevents rapid surface coagulation of proteins, improving kill efficiency.

List two advantages and one limitation of alcohol as a disinfectant.

Advantages: inexpensive, low toxicity. Limitation: does not reliably kill endospores.

Why is antibiotic development slowing despite clinical need?

Rapid microbial resistance and high R&D cost limit new antibiotic introductions.

Give two ideal properties of an antibiotic that are often hard to achieve.

Selective toxicity to pathogens only and resistance not easily acquired.

Why is using antibiotics inappropriate for routine surface sterilisation?

They are narrow-spectrum, costly, promote resistance, and do not kill all microbes on surfaces.

Which sterilisation method is preferred for heat-sensitive ophthalmic (eye) drops?

Membrane filtration.

Which sterilisation method is commonly chosen for powders that would clump if steamed?

Dry-heat oven sterilisation.

How are blood bags often sterilised without heating the contents?

Low-dose ionising (gamma) radiation.

What combination of method and concern makes moist heat ideal for most surgical instruments?

Autoclaving offers rapid, reliable sterilisation without chemical residues, though care must be taken to prevent corrosion.