microbes and normal flora

week 4 ctb

germ theory

• ancient Greeks thought infectious diseases were due to miasma (poisonous vapours from rotting materials)

• some diseases were known to be contagious

• humans and animals were known to have visible worm infestations

• 1674: antonie van Leeruwenhoek discovered micro-organisms

• 1700: few unsubstantiated theories about microbes causing disease

• 1847: ignaz semmelweis prevented post-natal sepsis by handwashing

• 1854: john snow linked cholera to contaminated water

• 1858: louis pasteur proved fermentation or spoilage due to microbes

• 1876: robert koch proved that a specific microbe caused anthrax

Koch’s postulates

1. micro-organism must be found in abundance in all organisms suffering from disease but should not be found in healthy organisms

2. micro-organism must be isolated from a diseased organism and grown in pure culture

3. cultured micro-organism should cause disease when introduced into a healthy organism

4. micro-organism must be re-isolated from the innoculated diseased experimental host and identified as being identical to original specific causative agent

classes of microbial organisms

viruses

bacteria

fungi

protozoa

prions

metazoa

viruses

• 20nm-1um in size

• RNA or DNA

• no independent metabolism and no organelles

• simple protein coat (+- cell membrane from host cell)

• eg: flu, rhinovirus, HIV, ebola, smallpox, herpes

bacteria

• 1-10 um in size

• DNA as a single chromosome (prokaryotes)

• independent metabolism and no organelles

• cell wall (distinct from cell membrane of host organism)

• peptidoglycan cell wall= gram stain +ve

• lipopolysaccharide capsule= gram stain -ve

bacteria replication

• replicate by asexual binary fission

• don’t live for very long unless they replicate

gram staining bacteria

• gram= positive or negative

• shape= coccus or bacillus/rod

• gram +ve cocci= Staphylococcus, Streptococcus

• gram +ve bacilli= Lactobacilli, Bacillus anthracis

• gram -ve cocci= Neisseria meningitidis, Haemophilus influenzae

• gram -ve bacilli= E coli, salmonella

fungi

• 5-10um in size

• DNA as multiple chromosomes in a nucleus (eukaryotes)

• independent metabolism and organelle (no chloroplasts)

• cell wall (distinct from cell membrane of host organism)

protozoa

• 10-100um in size

• DNA as multiple chromosomes in a nucleus (eukaryotes)

• independent metabolism and organelles 

• cell membrane (similar to host organism)

prions

• 10nm

• no RNA or DNA

• just a protein enzyme

• no independent metabolism and organelles

• no protein coat, cell wall or cell membrane

interactions between microbes and humans

• interactions can be symbiotic= close and long term interaction between 2 different species

• commensal= symbiotic relationship between 2 different species where one derives some benefit and other is unaffected

• colonisation= where a microbe grows on or in another organism without causing disease

• infection= invasion and multiplication of microbes in area of body where they are not normally present 

• pathogen= microorganism that is able to cause disease

• obligate pathogen= pathogen must cause disease in order to be transmitted from one host to another

• they must also infect a host in order to survive, in contrast to other pathogens that are capable of survival outside of host

• opportunistic pathogen= pathogen takes advantage of an opportunity not normally available such as a host with a weakened immune system (altered microbiota, disrupted gut flora, or breached igumentary barriers)

• parasite= pathogens that are protozoa or metazoa

interactions between microbes and humans cont.

• normal commensal bacteria= normal flora or microbiota

• they occuoy majority of available body areas such as skin, mouth, upper airways, lower airways, GI tract ahd genital tract

• least commensal and probably mutalistic in preventing more pathogenic bacteria from occupying those areas 

• antibiotic treatments can eliminate normal flora and make infections more likely

proving pathogen-disease link

• diagnosing infection doesn’t require all of Koch’s postulates

• depends on finding evidence of microbes in area of body where they’re not normally present

• they must correlate with clinical features of patient to check whether this diagnosis is plausible

• identification of microbes may be uncertain and take time, therefore correlation with clinical features is vital