ANSC20003 Chap.2 Bacterial Cells, Culture and Identification
Bacterial Cells, Culture and Identification
Small, everywhere, and not the enemy by default
Nothing alive and made of cells is smaller; they have no membrane around their genetic material, unicellular and self-replicating, dividing by binary fission.
Their genome is a single circular chromosome with plasmids carrying extra information, and their metabolic range extends to every naturally occurring organic compound, which is why they are central to decomposition and nutrient cycling.
In an animal the same versatility runs both ways: they train the immune system to distinguish pathogens from normal flora, compete with pathogens and produce antimicrobial substances, and they also cause many of the diseases on the international list of notifiable animal diseases.
What a bacterial name encodes
Classification uses phenotype, meaning appearance, colony and cell morphology, staining outcomes and biochemical reactions, and genotype, meaning DNA-based methods that establish evolutionary relationships.
Names frequently encode shape, a defining feature, a habitat, a nutrient, a discoverer or a disease.
Below the species line the distinctions still matter clinically: subspecies of one species can differ between a cause of disease and a commensal, and strains of one species can differ between a probiotic and a foodborne pathogen.
The wall decides the stain
There are no organelles inside, and the chromosome lies loose as a dense mass, so copying the genome and reading it into protein happen in a single compartment.
The cell wall carries a peptidoglycan layer whose thickness determines the Gram reaction.
The optional structures are the ones that matter for virulence: fimbriae for adhesion and colonisation, pili including the conjugative pilus that transfers DNA, flagella for motility, capsules that shield the cell and resist drying, and spores that survive heat, chemicals, drying and radiation.
Growing one organism out of a mixture
Identification needs a pure culture, which arises from a single cell and produces colonies uniform in size, shape, colour and texture.
Media are general purpose, enriched, selective or differential, and the plating techniques answer different questions: streaking isolates colonies, spread plating enables counting, and pour plating supports both aerobic and anaerobic growth.
What this chapter covers
- 01
What bacteria are, and where they sit relative to viruses, yeasts and eukaryotic cells
- 02
Normal flora and pathogens, and the conditions that turn one into the other
- 03
Binomial nomenclature, and what subspecies and strain distinctions change clinically
- 04
Cell structure, the peptidoglycan layer and the Gram reaction
- 05
Fimbriae, pili, flagella, capsules and spores as virulence and survival equipment
- 06
General purpose, enriched, selective and differential media
- 07
Streaking, spread plating and pour plating, and the colony and cell description vocabulary
Deciding what three plates have established
- 1Say what each medium selects for or differentiates on.
- 1Combine the three results into a single statement.
- 1Name the limit of the combined result and the step that removes it.
Key terms
- Binary fission
- Asexual bacterial division producing two daughter cells identical to the parent except where a mutation has occurred.
- Peptidoglycan
- The cell wall polymer whose thickness differs between Gram positive and Gram negative bacteria and determines the stain result.
- Pure culture
- A culture arising from a single cell, producing colonies uniform in size, shape, colour and texture.
- Enriched medium
- A medium carrying extra nutrients so that organisms with demanding growth requirements will grow on it.
- Endospore
- A dormant structure formed inside some bacterial cells that resists heat, chemicals, drying and radiation and preserves the genome in the environment.
Bacterial Cells, Culture and Identification FAQ
What makes a commensal become a pathogen?
The definition of normal flora includes the word healthy. The same organisms can become opportunistic pathogens when the health status of the animal changes, or when a wound gives them access to a site where they do not belong, so the shift is about circumstance rather than about the organism acquiring something new.
Why does the Gram stain work?
Gram positive cells carry a thick peptidoglycan layer that retains the first dye through decolourisation, while Gram negative cells carry a thin layer beneath an outer membrane and lose it, taking up the counterstain instead. The colour is therefore a report on wall structure rather than a property in its own right.
What is the difference between a selective and a differential medium?
A selective medium is built so that only part of what is on the swab will grow, so the information sits in whether anything came up at all. A differential medium contains substrates and indicators that reveal a biochemical difference, so the information is in what the growth looks like. A single plate can do both jobs at once.
Why does identification require a pure culture rather than the original sample?
Because every phenotypic descriptor assumes one organism. Colony morphology, cell shape, arrangement and a biochemical reaction all become ambiguous on a mixture, and two colony types on a plate mean two organisms whose results cannot be attributed.
Exam move
Build one table for media and one for structures and rehearse them by writing rather than reading. For each medium, be able to state what it selects for and what it differentiates on in a single sentence, because that phrasing is what practical answers are marked against.
For the Gram reaction, learn the direction with a memory hook and then check it against a known organism, since a reversed direction is the most expensive single error in this block. Practise describing a colony using the permitted terms rather than adjectives of your own.
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