Lecture 11: introduction to microorganisms Flashcards
Cell types
> Prokaryotes
-Bacteria
-Archaea
Eukaryotes
-All cells apart from bacteria and archaea
The major bacterial groups
> The Bacteria are divided into 23 groups or ‘phyla’.
The largest group is the Proteobacteria.
It is an extremely diverse group - most physiological types are represented and the wide range of morphological forms that they possess gives rise to the group’s name. (Proteus was a mythological Greek god who was able to assume many different forms.)
All members of the group are ‘Gram-negative’.
The gram stain
> The Gram stain is an example of a ‘differential stain’ i.e. it does not stain all kinds of cells the same. On the basis of their reaction to the Gram stain, bacteria can be divided into two major groups, ‘Gram-positive’ and ‘Gram-negative’.
After staining, Gram-positive bacteria appear purple and Gram-negative bacteria appear red.
This difference in reaction arises because of differences in the cell wall structure and composition of Gram-positive and Gram-negative cells.
The Proteobacteria
> Currently more than 460 genera and 1600 species have been identified, all of which are ‘Gram-negative’ and representing almost half of all accepted bacterial genera.
The group includes well-known species such as
-Escherichia coli,
Pseudomonas aeruginosa,
Acetobacter species and
Campylobacter jejuni.
It also includes the nitrogen-fixing bacteria, Rhizobium and Azotobacter.
The ‘gram-positive’ bacteria
> These bacteria include genera such as Bacillus, Clostridium, Lactobacillus, Streptococcus and Mycoplasma.
The actinomycetes, also, belong to this group.
The best known and most studied member of which is the genus Streptomyces.
Importance of fungi
> Production of antibiotics (e.g. penicillins), cyclosporins (immunosuppressants in transplant surgery), modification of steroids (for contraceptives)
‘Cell factories’ for the production of foreign gene products e.g. yeast production of hepatitis B vaccine
Decomposition of complex plant and animal remains in natural environments such as soil
Rotting of food, textiles etc. etc. including timber e.g. dry rot caused by Serpula lacrymans
All of the above processes are carried out by ‘saprotrophic’ fungi
There are also ‘Parasitic’ (pathogenic) fungi, which cause diseases of plants and animals and ‘Symbiotic’ fungi, which form lichens and mycorrhizas
Fungal parasites of plants
Fungi are the most important causes of crop diseases, responsible for billions of pounds worth of damage each year and for periodic devastating disease epidemics.
Fungal pathogens of humans
> In contrast to the many fungal parasites of plants, there are only about 200 fungi that infect humans or other warm-blooded animals.
In fact, humans have a high degree of innate immunity to fungi, with the exception of the dermatophytic fungi, which commonly cause infections of the skin, nails and hair.
With the advance of AIDS and the increasing role of transplant surgery, however, fungi are becoming one of the most significant causes of death of immunocompromised and immunosuppressed patients.
There are few antifungal drugs available to treat the major fungal infections of man.
Magic mushrooms/ liberty caps
> They are not uncommon, prefer disturbed ground, and are often found in parks, gardens, construction sites, as well as in forests and fields around the world.
Also sold, along with others i.e. A. muscaria, in e.g. Amsterdam
Mushroom poisoning
> Although dramatic and tragic, death from mushroom poisoning occurs in the minority of cases.
Last year the National Poisons Information Service stated there were 257 cases of poisoning linked to eating mushrooms.
Most people who eat a poisonous mushroom suffer an unpleasant but relatively mild and short-lived bout of poisoning.
The quicker the symptoms arise, the less severe the attack. The toxins of deadly mushrooms are slow acting and do not cause symptoms for 6 – 12 hours or more after eating.
Those causing milder poisonings make people sick in two hours or less. After a few hours of discomfort (nausea, vomiting), the victim recovers completely.
Symbiotic associations
> Lichens
-Symbiotic association between fungus + alga/cyanobacterium Alga or cyanobacterium provides C-compounds
-Fungus provides protection, water, minerals
> Mycorrhizas e.g. ectotrophic mycorrhiza
-Associated with roots of trees e.g. beech, pine
-‘Fungal sheath’ is the nutrient absorbing surface for the root
-The fungus increases the uptake of N, P, and K in nutrient-poor soils and in return gains C-cpds (sugars) from the tree
Characteristics of the fungi
> Heterotrophic for carbon
Eukaryotic cell structure
Filamentous in most groups
Cell wall present
Spores produced in reproduction
Non-motile except in reproduction in some fungi
Filamentous
> Each filament is called a hypha (pl. hyphae)
Network of branched hyphae is called the mycelium
Cell wall present
> The walls of all fungi contain a mixture of fibrillar and amorphous or matrix components.
The fibrillar components are normally composed of chitin b(1-4) linked polymer of N-acetyl-glucosamine and form a network which confers rigidity to the wall.
The amorphous components include proteins, mannans (polymer of mannose) and glucans (b(1-3), b(1-6) and a(1-3) linked polymers of glucose).
Septation
> In the majority of fungi, the hyphae as they grow apically, become divided, at intervals, by transverse walls or septa into compartments.
Two large divisions within the fungi
regularly septate – ‘higher fungi’
aseptate – ‘lower fungi’
Spore produced in reproduction
> Fungi produce spores for :-
Dispersal and Survival
There are 2 major types of spores
-Sexual spores,
»_space;which are thick-walled and resistant to unfavourable environmental conditions
-Asexual spores,
»_space;which are produced in very large numbers for dispersal but normally have low resistance to unfavourable conditions
Fungal spores- asexual
> Sporangiospores inside a sporangium e.g. Mucor
Conidia (sing. conidium) e.g. Cladosporium conidia in chains
Paecilomyces – conidia (sing. conidium) in chains
Fungal spores- sexual
> Mucor - zygospores
Sordaria – ascospores inside an ascus (pl. asci) enclosed within a fruitbody (perithecium)
Basidiospores on basidium (pl. basidia)
Important groups of fungi- ‘lower fungi’
> Chytridiomycetes e.g. Allomyces
-Asexual reproduction – motile sporangiospores (zoospores)
Zygomycetes e.g. Mucor
-Asexual reproduction – non-motile sporangiospores
-Sexual reproduction – zygospores
Important groups of fungi- ‘Higher fungi’
> Ascomycetes e.g. Sordaria
-Asexual reproduction – conidia
-Sexual reproduction – ascospores
Basidiomycetes e.g. Agaricus
-Asexual reproduction – conidia
-Sexual reproduction – basidiospores
Mitosporic fungi e.g. Penicillium
-Asexual reproduction – conidia
-Sexual reproduction – absent
