MI1.{1-2,10,12-13} | General Microbiology I: History, Morphology, Sterilisation & Staining — Glossary

3% hydrochloric acid in 70% ethanol; the decolouriser in the ZN stain that removes carbolfuchsin from non-acid-fast organisms while mycobacteria retain it due to their waxy wall.

Bacteria with a mycolic acid-rich cell wall that, once stained with hot carbolfuchsin, resist decolourisation by acid-alcohol; Mycobacterium tuberculosis is the most clinically important AFB.

A surface molecule on a pathogen that mediates attachment to specific host cell receptors, the first step in colonisation and infection.

Historical term for microorganisms first visualised by Leeuwenhoek; now superseded but contextually important in the history of microbiology.

Application of an antimicrobial agent to living tissue (skin or mucous membranes) to prevent or arrest infection; the chemical used is called an antiseptic.

A pressurised steam steriliser that achieves sterilisation at 121°C and 15 psi for 15 minutes; the gold standard for heat-stable materials in clinical settings.

The total number of microorganisms present on a surface or in a material before sterilisation; must be reduced by cleaning (pre-sterilisation) to ensure reliable sterilisation.

A structured community of bacteria encased in a self-produced extracellular matrix adherent to surfaces; biofilms are resistant to antibiotics and host defences, commonly forming on catheters and prosthetic devices.

A preparation of highly resistant bacterial spores (Geobacillus stearothermophilus for autoclaves) used to verify that sterilisation conditions were lethal; non-growth after incubation confirms sterilisation.

A chemical test performed daily in pre-vacuum autoclaves to verify adequate steam penetration throughout the load, using a specific indicator sheet placed in a standard test pack.

An outer polysaccharide layer surrounding some bacteria that resists phagocytosis, a key virulence factor in organisms like Streptococcus pneumoniae and Klebsiella pneumoniae.

The primary stain in the ZN technique; a phenol-fuchsin dye that penetrates the mycolic acid wall of mycobacteria when applied with heat; retained against acid-alcohol decolourisation.

A bisbiguanide antiseptic with prolonged residual activity on skin; used for surgical hand antisepsis, central venous catheter site care, and pre-operative skin preparation.

A crystalline RNA-protein body present in immature E. histolytica cysts; rod-shaped with rounded ends; disappears in mature cysts; distinguishes immature from mature cysts.

The primary stain in the Gram technique; forms a complex with iodine inside the bacterial cell; retained in Gram-positive organisms and washed out of Gram-negative organisms during decolourisation.

The dormant, non-motile, environmentally resistant form of a protozoan parasite; responsible for transmission; identified by characteristic nuclear number and internal structures on iodine staining.

In Gram staining, the acetone-alcohol solution that removes the CV-I complex from Gram-negative bacteria through dissolution of the outer membrane; over-application causes false Gram-negative results.

A fungus that exists as a mould at ambient temperature (25°C) and converts to a yeast form at body temperature (37°C); examples include Histoplasma capsulatum and Sporothrix schenckii.

Elimination of most or all pathogenic microorganisms (but not necessarily all spores) from inanimate surfaces or objects.

Disruption of the normal composition or diversity of the microbiome, associated with conditions such as C. difficile colitis and inflammatory bowel disease.

A metabolically dormant, highly resistant form produced by Bacillus and Clostridium spp. that survives heat, desiccation, and many disinfectants; requires autoclaving to destroy.

Lipopolysaccharide (LPS) component of the outer membrane of Gram-negative bacteria; released on bacterial lysis and triggers systemic inflammatory response (sepsis).

The fungal cell membrane sterol, analogous to human cholesterol but structurally distinct; the target of antifungal drugs such as amphotericin B and azoles.

A gaseous chemical sterilant that alkylates nucleic acids and proteins; used for heat-labile medical devices; requires 12-24 hour aeration to remove toxic residues.

A cell with a membrane-bound nucleus and membrane-bound organelles; includes fungi, protozoa, helminths, and all human cells.

A protein toxin actively secreted by bacteria (typically Gram-positive); highly specific in action and immunogenic — examples include cholera toxin and tetanus toxin.

A stool concentration technique using formalin and ether/ethyl acetate centrifugation to sediment cysts, ova, and oocysts; increases diagnostic sensitivity when the direct wet mount is negative.

A thermophilic, spore-forming bacterium used as the biological indicator for autoclaves; its spores are the most heat-resistant standard test organism.

Pasteur's foundational concept that specific microorganisms cause specific infectious diseases, replacing the miasma theory of spontaneous origin.

A dialdehyde chemical disinfectant/sterilant used at 2% for high-level disinfection of heat-sensitive items like endoscopes; sporicidal with prolonged (10-hour) exposure.

A differential staining technique that classifies bacteria as Gram-positive (purple) or Gram-negative (pink) based on the ability of their cell wall to retain the crystal violet-iodine complex after decolourisation with acetone-alcohol.

Rapid passage of a glass slide through a Bunsen flame to kill microorganisms and adhere the smear to the slide surface; essential before any staining procedure.

High-Efficiency Particulate Air filter that removes 99.97% of airborne particles ≥0.3 µm; used in laminar flow cabinets, OT ventilation, and immunocompromised patient isolation rooms.

Destruction of all vegetative bacteria, most fungi, viruses, and mycobacteria, but not necessarily all bacterial spores; achieved by agents such as 2% glutaraldehyde or OPA.

A dry-heat steriliser operating at 160°C for 60 min or 180°C for 30 min; used for glassware, metal instruments with sharp edges, and materials that would be damaged by moisture.

Four criteria establishing causality between a microorganism and a disease: isolation from every case, pure culture, disease reproduction in healthy host, and re-isolation from the experimental host.

An iodine-potassium iodide solution used in stool examination to stain glycogen (brown) and nuclei of protozoan cysts, aiding identification of nuclear number and chromatoid bodies.

The irregular, wavy, albuminous outer coat of Ascaris lumbricoides fertile eggs seen on stool microscopy; a distinctive identification feature of Ascaris ova.

The collective community of microorganisms (bacteria, fungi, viruses) that colonise a host in a commensal or mutualistic relationship without causing disease under normal conditions.

A substance that fixes a stain within a cell; Gram's iodine is the mordant in the Gram stain, forming the insoluble crystal violet-iodine complex.

A long-chain fatty acid unique to the mycobacterial cell wall that confers hydrophobicity, acid-fastness, and resistance to many antibiotics and host defences.

National Tuberculosis Elimination Programme standardised grading for ZN smear positivity: negative, scanty (1-9 AFB/100 fields), 1+ (10-99/100 fields), 2+ (1-10/field), 3+ (>10/field).

An organism that can only replicate inside a living host cell because it lacks its own metabolic machinery; viruses and Chlamydia are examples.

A mild heat treatment (63°C/30 min or 72°C/15 sec) that kills vegetative pathogens in liquids like milk without sterilising; prevents transmission of Mycobacterium bovis, Brucella, and Salmonella.

The qualitative ability of a microorganism to cause disease in a susceptible host.

A mesh-like polymer of sugars (NAG-NAM) and amino acids forming the bacterial cell wall; thicker in Gram-positive organisms and targeted by β-lactam antibiotics.

Extrachromosomal circular DNA in bacteria that often carries genes for antibiotic resistance, virulence factors, and metabolic pathways; transmissible between bacteria by conjugation.

A cell lacking a membrane-bound nucleus; genetic material is in the cytoplasm as a nucleoid. Bacteria and Archaea are prokaryotes.

Cationic surfactant disinfectants that disrupt lipid membranes; effective against Gram-positive organisms but not Gram-negative bacilli, mycobacteria, or spores; used for routine surface cleaning.

The red/pink counterstain in the Gram technique that stains decolourised Gram-negative bacteria and host cells, providing contrast against the purple-stained Gram-positive organisms.

An oxidising disinfectant (bleach) used at 0.1% for routine surfaces, 0.5% for blood spills, and 1% for C. difficile environmental decontamination; sporicidal at high concentrations.

A risk-based framework classifying medical devices as critical (penetrate sterile tissue — must be sterilised), semi-critical (contact mucous membranes — high-level disinfection), or non-critical (contact intact skin — low/intermediate disinfection).

The complete destruction or removal of all viable microorganisms, including bacterial endospores and prions, from an object or surface.

The active, vegetative, motile feeding form of a protozoan parasite; trophozoites are fragile and must be examined in fresh stool preparations within 30-60 minutes of collection.

A quantitative measure of pathogenicity, reflecting the severity of disease or the minimum infective dose required.