3.4: Microbiology

Coccus

Spherical shaped bacteria, such as Staphylococcus.

Bacillus

Rod-shaped bacteria, such as E.coli.

Spiral

Corkscrew-shaped bacteria, such as Spirillum.

Single, pairs, chains, clusters

The four categories of grouping.

Gram stain

Test which shows whether a bacteria has a lipopolysaccharide layer in the outer cell membrane.

Negative

Bacteria that is red at the end of the gram stain reaction - has an extra lipopolysaccharide layer, and a thinner peptidoglycan cell wall.

Positive

Bacteria that is purple at the end of the gram stain reaction, does not have an extra lipopolysaccharide layer, and has a thicker peptidoglycan cell wall.

Lipopolysaccharide

Layer which prevents penicillin, lysozymes, and dye attaching to bacteria. In bacteria with a more chemically complex cell wall, and located in the outer cell membrane. These bacteria also have a thinner cell wall.

Peptidoglycan

Cell wall molecules used by bacteria, also known as murein. Gives the cell shape, and prevents cell damage caused by osmosis.

Crystal violet

Dye used at the start of the gram stain reaction to dye all bacteria purple.

Lugols iodine

Used to bind crystal violet to peptidoglycan more strongly - acting as a mordant.

Mordant

Term for a substance that fixes a dye to a material. In the gram stain reaction, this is Lugol’s iodine.

Acetone-alcohol

Used to remove unbound CV from gram negative bacteria, leaving it transparent. Known as the decolouriser.

Safarin

Used to dye the remaining gram negative bacteria red, which is transparent after decolourisation. The counterstain.

Lysozyme

An antibacterial enzyme in human tears and saliva, which hydrolyses peptidoglycan bonds.

Nutrient broth

Type of nutrient media that is made of liquid.

Nutrient media

Supplies nutrients to bacteria during culturing. Can be liquid or solidified using agar. Contains a carbon and energy source (normally glucose) and organic and inorganic nitrogen for amino acid synthesis.

Vitamins

One of the bacterial growth factors, including biotin.

Mineral salts

One of the bacterial growth factors, including sodium and magnesium.

Obligate aerobes

Term for organisms which require oxygen such as Mycobacterium tuberculosis.

Facultative aerobes

Microorganisms that do better with oxygen, but can survive in its absence, such as E.coli.

Obligate anaerobes

Microorganisms that respire without oxygen and cannot grow in its presence, such as Clostridium bacteria.

Aseptic

Technique to avoid contamination of the environment and of the culture with microorganisms. Done using sterilisation before and after use, sterile loops, flaming culture vessels, constant use of bunsen burners and taping petri dish lids shut.

Inoculation

When bacteria is introduced. To maintain aseptic technique, culture vessel lids are held and the vessels are heated, inoculation loops are heated and petri dish lids are left as close to shut as possible.

Pathogenic

Harmful bacteria, avoided in culture by heating to 25 degrees instead of 37 degrees, and by not sealing the lid completely to promote anaerobic bacterial growth.

Autoclaves

Sealed containers used in a lab environment where glass and metal are heated to 121 degrees under pressure for 15 minutes. Disposable materials are placed in autoclavable plastic bags and subjected to the same process before disposal.

Direct

Method of measuring bacterial growth where individual cells are counted, either only living or both living and dead. Methods include serial dilution and haemocytometers.

Indirect

Method of measuring bacterial growth where individual bacteria colonies are not counted. One method is turbidimetry.

Viable

Type of direct bacterial growth measurement that counts only living cells. One method is serial dilution.

Total

Type of direct bacterial growth measurement that counts both living and dead cells. Methods include haemocytometers and turbidimetry.

Serial dilution

Method of direct bacterial growth measurement where dilution is done multiple times. All samples are spread over an agar plate and incubated, and only one with 20-100 viable colonies is counted. A viable count.

Haemocytometer

Most accurate method of direct bacterial growth measurement. A specialised microscope with a raised platform is used. A total count.

Incubation

Occurs after inoculation, when a culture is allowed to grow.

Turbidimetry

A colorimeter is used to measure the turbidity of the culture, which increases alongside cell count. To find a total count, the exact absorption of a suspension is compared to a graph of light suspension and bacterial cell count. This is a total count, and can be done in the field.

Convection

Current that moves airborne particles away from the source of the heat, and therefore maintains aseptic technique when heating bottles and leaving bunsen burners on.