Microbiology Flashcards
How many bacteria are said to be in 1cm^3 of freshwater?
How many bacteria are in 1g of soil?
How many bacteria does out microbiome contain?
(How many different species)
- 1 million
- 40 million
- 10^14
500-100 different species
Name some of the differences in physical properties between bacteria
- Size
- Cell wall structure
- Staining characteristics
- shape
What do their genetic differences produce?
- Different metabolic features
- Different surface molecules
- Different antigenic properties
What are the smallest bacteria and how small are they?
What are the largest bacteria and how large are they?
What is the size of E.coli
]-The smallest are Archaea, such as Nanoarchaeum equitans –) 0.4 μm
]- The largest bacterium is the sulphur bacterium Thiomargarita namibiensis –) 750μm diameter
- E.coli is 1.8μm diameter and 7μm long
A genus of bacteria has one of three main shapes, and the shape is sometimes indicated in its name:
When is further differentiation possible?
]- Bacillus or rod-shaped, e.g Eschericha e.g Eschericha; Bacillus
]- Coccus or spherical, e.g Staphylococcus; Streptococcus
]- Spiral or corkscrew-shaped, e.g Spirillum
Further differentiation is often possible according to the way bacteria tend to group. They may be single, e.g. Helicobacter; in pairs, e.g Diplococcus pneumonia; in chains, e.g Streptococcus or in clusters, e.g. Staphylococcus;
Explain gram staining
What colour are Gram-positive bacteria after staining?
What colour are Gram-negative bacteria after staining?
- The gram stain allows microbiologists to distinguish between Gram positive and Gram negative bacteria
- The different staining properties are due to differences in the chemical composition of their cell walls
- Before staining, bacteria are colourless
- After staining, Gram-positive bacteria are stained violet
- After staining, Gram-negative bacteria are stained red
Explain the characteristics of bacterial cell walls
- The cell wall of all bacteria is a 3-dimentional network of polysaccharides and polypeptides, know as peptidoglycan or murein
- The cross-linking of these molecules provides strength and give the cell its shape
- The wall protects against swelling and bursting or lysis caused by the osmotic uptake of water
- Gram-positive bacteria possess this basic cell wall structure and Gram-negative bacteria have an additional outer layer of lipopolysaccharide
What are the four stages of Gram staining?
- Crystal violet (BASIC DYE)
Binds to peptidoglycan so all bacteria stain purple - Lugol’s iodine (MORDANT)
Binds the crystal violet to the peptidoglycan more strongly - Acetone-alcohol (DECOLORISER)
Removes unbound crystal violet and lipopolysaccharide:
]-Gram- negative bacteria lose their stain and become colourless
]- Gram positive bacteria remain purple - Safranin (COUNTER-STAIN)
]- Gram-negative bacteria stain red
]- Gram-positive bacteria remain purple
Explain Gram-positive bacteria with regard to staining
- After staining, Gram-positive bacteria are violet or purple under the microscope. They include Bacillus, Staphylococcus and Streptococcus
- The absence of an outer lipopolysaccharide layer in the cell walls of Gram-positive bacteria allows them to bind stain efficiently and makes them more susceptible to the antibiotic, penicillin, and the enzyme, lysosome, than Gram-negative bacteria
- Bacteria constantly make and break chemical links in their cell walls. The antibacterial enzyme lysozyme which occurs in human tears and saliva, hydrolyses the bonds holding the peptidoglycan molecules together
- Penicillin prevents the bonds inter-linking peptidoglycan molecules from forming. This is especially significant when the bacteria make new cells when they divide. Penicillin therefore makes the cell walls structurally weak and prone to collapse. Water uptake by osmosis bursts the cell
Explain Gram-negative bacteria with regard to staining
- Gram-negative bacteria have a more chemically complex cell wall than Gram-positive bacteria. Their outer membrane is supplemented with large molecules of lipopolysaccharide which protects the cell and exclude dyes like crystal violet, so they appear red or pink in the cell
- Gram-negative bacteria include Salmonella specie and E.coli. The outer lipopolysaccharide protects the peptidoglycan below and so they are not affected by lysosome and are resistant to penicillin. To control them requires a different class of antibiotics, that interfere with the cells ability to make proteins. Eukaryotic cells also make proteins, but the protein making cellular machinery is different from that in bacteria, so these antibiotics do not harm them
Conditions necessary for culturing bacteria
- Micro-organisms can undergo fission and reproduce quickly, given a suitable environment
- In optimum conditions they divide every twenty minutes
- In the laboratory bacteria can be grown on a wide variety of substrate providing they are supplied with nutrients, water and suitable physical conditions, such as temperature
- Micro-organisms vary in their requirements and usually grow over a range of temperature and pH values with an optimum within the range
Micro-organisms require the following conditions for growth: Nutrients
Nutrients- in the laboratory, nutrients are supplied in nutrient media
The bacteria may be cultured in a liquid medium, called a nutrient broth, or on medium solidified with agar. The media provide water and they include:
- A carbon and energy source, usually glucose
- Nitrogen for amino acid synthesis, in organic molecules and in inorganic forms such as nitrate ions
Micro-organisms require the following conditions for growth: Temperature
AS bacterial metabolism is regulated by enzymes, the range of 25 -45°C is suitable for most bacteria. The optimum for mammalian pathogens is around 37° C the temperature of the human body
Micro-organisms require the following conditions for growth: pH
Most bacteria mare favoured by slightly alkaline conditions (pH 7.4), whereas fungi grow better in neutral to slightly acid conditions
Micro-organisms require the following conditions for growth: Oxygen
- Many micro-organisms require oxygen for metabolism and are obligate aerobes, e.g. Mycobacterium tuberculosis
- Some grow best in the presence of oxygen but can survive in its absence; these are facultative anaerobes, e.g. E.coli
- Others cannot grow in the presence of oxygen and are obligate anaerobes
- Clostridium bacteria are obligate anaerobes that produce toxins or poisons in a wound
- They destroy body tissue in the condition called ‘moist gangrene’
