Makindo Medical Notes.com

Introduction

• Single celled. Contain apparatus for replication and division
• Bacteria are everywhere. Size is about 2 micrometres
• Have chromosome, ribosomes, cytoplasm, and an outer membrane
• Prokaryotes. Contain DNA no nucleus
• Reproduce by binary fission. has DNA polymerase

Rigid cell wall

• Protective - osmosis/infection
• Peptidoglycan - N acetylglucosamine and N-acetylmuramic acid

Staining

• Gram positive - thick peptidoglycan cell wall which may contain antigens (Lancefield) surrounded by a capsule. Retains purple colour of crystal violet and iodine despite decolourisation with acetone or alcohol.
• Gram negative - periplasmic space then thinner peptidoglycan cell wall and then lipoprotein and lipopolysaccharide. Loses purple colour of crystal violet and iodine despite decolourisation with acetone or alcohol. Pink discolouration with carbol fuschin.
• Ziehl-Neelson - used for identifying mycobacteria which have high amounts of mycolic acid and high lipid content in the cell wall. It stains red with carbol fuschin which cannot eb removed by 20% sulphuric acid and alcohol. "acid and alcohol fast"
• Auramine - used for mycobacteria which stain and fluoresce despite exposure to acid

Cell membrane

• Semipermeable membrane with proteins in a lipid bilayer
• Chromosomal DNA attaches to it

Shape

• Spherical - cocci
• Cylindrical - rods
• Helical - spirochaetes

Arrangement

• Chains - streptococci
• Clusters - staphylococci, pneumococci
• Angled - corynebacteria

External structure

• Flagella - long filaments, rotate, cause motion
• Fimbriae/pili - short filaments composed of pilin used in the transfer of genes from cell to cell
• Capsules - polysaccharide protective inhibiting phagocytosis increasing virulence

Internal structure

• Mosesomes - cytoplasm, DNA
• Chromosome - dsDNA coiled with RNA core
• Ribosomes - protein syntheses. Types 30S and 50S
• Cytoplasmic inclusions - stores
• Plasmids - circular fragments of DNA

Bacterial division

• Every 30 minutes
• Depends on temperature, nutrients

Genetics

• Chromosomal DNA - circular dsDNA attaches to cell membrane
• Plasmids - smaller and replicate independently
• Mutations occur with faults in DNA replication
• Conjugation: Gene transfer of plasmids which are small round strands of dsDNA. R-plasmids are important in antibiotic resistance. Can transfer between bacteria.
• Transduction: Bacteriophages are viruses that affect bacteria. Integrate into the bacterial chromosome. Can very rarely transfer bacterial or plasmid DNA from one bacterium to another.
• Transposition: swapping of DNA between bacterial and plasmid DNA or plasmid to plasmid

Metabolism

• Often require pre-made molecules from host
• Requires elements - C,H,O,N, P, S, Ca, K, Mg, Fe
• Trace elements - Cu, Co, Mn, Mb, Zn often for enzymic function
• Amino acids - bacteria can break down protein
• Purines and pyrimidines
• Vitamins

Environment

• Water is essential
• Carbon dioxide - needed by all bacteria and a product of metabolism.
• Temperature - pathogens tend to prefer 37C
• pH of 7.2-7.4 is preferred

Incubation

• Aerobic : grow in air
• Microaerophilic: grow in air + 10% CO₂
• Anaerobic: grow with oxygen removed replaced with hydrogen and CO₂
• Facultative anaerobes - grow with aerobic or anaerobic conditions

Culture

• Water, NaCl, Peptone (protein), Meat or yeast extract, Blood (defibrinated horse blood)
• Solid - sterile agar jelly in Petri plates
• Blood agar - 10% horse blood. Bacterial colonies can show surrounding haemolysis alpha, beta or partial.
• Chocolate agar - heated blood agar due to red cell lysis. For fastidious bacteria
• Selective media - may contain inhibitors of unwanted bacterial growth
• Indicator media - lactose with pH indicator
• Liquid media e.g. Robertson's meat medium. Grows aerobic and anaerobic.
• Blood culture bottles - one has broth for aerobes and the other for anaerobes