DISTINGUISHING CHARACTERISTICS OF MICROBES Flashcards
Free-living forms of bacteria
Saprophytic: exist on dead organic matter – photosynthetic
Parasitic forms of bacteria
are of immense importance from a medical or veterinary standpoint because they survive on living cells causing damage and resulting in a disease condition
Mycoplasmas are
the smallest free-living cells
Very similar to bacteria; usually classified under bacteria
Differences between mycoplasma and bacteria include:
- Mycoplasmas are smaller (0.2 - 0.8 μm); can pass through bacteria-stopping filters
- Have the smallest genome size; lack many metabolic pathways and require complex media for their isolation
- Lack a rigid cell wall and are thus pleomorphic
- Completely resistant to penicillin (which acts by interfering with cell wall synthesis) but highly susceptible to tetracycline
Characteristics of Mycoplasmas
• Mycoplasmas are facultative anaerobes, except for M. pneumoniae, which is a strict aerobe
• Smallest and simplest organisms capable of autonomous growth
• Grow slowly by binary fission
• Occur both as saprophytes and as parasites
• Affinity for cell membranes and cause diseases
VIRUSES
Viruses lack independent metabolism
They are, therefore, obligate intracellular parasites that multiply only inside living cells.
The smallest infecting unit of the virus is called the
virion
How do viruses infect
Once inside a living cell, the virion directs the host cell metabolic machinery to synthesize infective virions which mature and are released, following the rupture of the cell, to infect surrounding cells.
Viruses that infect bacteria are known as
bacteriophages or phages.
RICKETTSIAE
Rickettsiae (singular = rickettsia) are obligate intracellular parasites of arthropods such as lice, mites, ticks and fleas.
Many are pathogenic for man and other animals to whom/which they are transmitted by the bites or in the excreta of the arthropod vectors.
They are thought to be intermediate between bacteria and viruses because they have features common to both.
Rickettsiae are able to make all the metabolites necessary for growth, but they have an ATP transport system that allows them to use host ATP. Thus, they are energy parasites as long as ATP is available from the host.
Rickettsiae are non-motile, non-sporeformers, and are readily destroyed by heat (heat labile), dehydration and ordinary antiseptics and antibiotics
PROTOZOA
unicellular non-photosynthetic eukaryotic micro- organisms that lack cell wall
large micro-organisms (2 – 100 μm in diameter); generally colourless.
cell membrane ranges in complexity and rigidity: flexible in amoebas, stiff pellicle in ciliates, which preserves a characteristic cell shape
Most are free-living and are found in a wide range of habitats including freshwater, marine, soil etc.
Some are parasitic in animals and man and are mostly haemotrophic (blood-borne).
Ingest particulate matter by phagocytosis
Some protozoa literally swallow particulate matter using a special structure called a gullet.
Fungi
They possess rigid cell walls
• differ from bacteria in having much larger cells
and in being eukaryotic.
• They are basically non-motile, except for their
flagellate spores.
• They reproduce either sexually or asexually (e.g. budding in yeast).
• Fungi digest food outside their bodies by releasing enzymes into the surrounding environment and absorbing the digested products.
• Many grow as saprophytes and recycle dead organic matter into useful nutrients for plants.
• some are parasites, and about 400 species are known to be pathogenic for man, animals and plants.
Fungi can be divided into four main groups:
- Yeasts: These are round or oval bodies that reproduce by budding, e.g. Saccharomyces cereviseae and Cryptococcus neoformans.
- Moulds (Filamentous fungi): These grow as filaments (hyphae) which interlace to form a meshwork called mycelium which can be easily seen without a microscope. Some of the hyphae terminate in rounded forms called conidia
- Yeast-like fungi: These grow mainly as yeasts, but may also grow in the form of chains of elongated, flamentous cells (pseudomycelium) which give rise to yeast cells by budding, e.g. Candida albicans, and Pityrosporum orbiculare.
- Dimorphic fungi: These have a filamentous form (saprophytic phase) when growing at room temperature, but when growing in the body (or at 37oC) they have a yeast form (parasitic phase), e.g. Histoplasma and Blastomyces.
What are prions
infectious self-reproducing protein structures
What microbes are responsible for transmissible spongiform encephalopathy tse diseases
Prions
Prions consist mainly of a specific protein called
PrP prion protein
