MICROORGANISMS

  • Microorganisms are so tiny that they are generally invisible to the naked eye. They can be seen only with the help of a microscope.
  • Their presence on this planet is universal as they are found in almost every part of natural and man made environment including the food we eat.
  • The information gathered can be utilized not only to prevent the growth of harmful microorganisms during cooking, processing, storage and service but also to optimize beneficial microbial fermentations.
  • The food we eat is not sterile. Microorganisms are present in food partly due to the natural microflora of raw material and additionally due to contamination during harvesting/slaughtering, processing, storage and distribution.

EXAMPLES OF MICROORGANISMS

  • Fuzzy, cottony growth on a slice of bread
  • Ropiness in yoghurt
  • Brown, soft spots in stored apples
  • Bulged can of fish.
  • Porous texture of bread
  • Blue colour in Roquefort cheese
  • Clean acid flavour of sauerkraut
  • Sourness and effervescence in kefir

The microorganisms are categorized (types of microorganisms) according to their

  • Size,
  • Shape and
  • Structure

TYPES OF MICROORGANISMS AND THEIR REPRODUCTION:

  • Bacteria
  • Mold
  • Yeasts
  • Viruses
  • Protozoans
  • helminths

BACTERIA:

Bacteria, the most abundant of all organisms, are unicellular, relatively small in size and have varied shapes.

  • They are found in many morphological forms:
  • Spherical (cocci),
  • Rode shape (bacilli)
  • Curved rods (comma) and
  • Spiral (spirochetes).
  • The cocci and bacilli can exist as such or form associations. The cocci can form:

    • Cluster (staphylococcus),
    • Chains (streptococcus) or
    • Tetrad (micrococcus)
    • Amond rods, E coli and Salmonella are short, whereas clostridium is large. Vibro is an example of curved rods.
    • Some bacteria can form endospores (spores) in adverse environment. The spores are metabolically dormant. The spore forming bacteria are:
    • Clostridium
    • Bacillus
    • Are significant in causing foodborne illness.

REPRODUCTION OF BACTERIA:

  • Bacteria usually reproduce asexually by binary fission at a fast pace.
  • Their cell multiply at a constant rate depending on the extrinsic environmental conditions.

GROWTH CURVE:

There are four phases: 

  • LAG PHASE: the cell adjust to the new environment and there is no increase in their numbers in this initial phase.
  • EXPONENTIAL (LOG) PHASE: the cells double by binary fission at regular interval. Their growth is 1,2,4,8,16 and so on, the growth rate is highest during this phase.
  • STATIONARY PHASE: the rate of cell division decreases due to the exhaustion of available nutrients and accumulation of end products. The death rate equals growth rate and the number of bacteria remains constant during this phase.
  • DEATH PHASE: the viable cells start dying in an exponential manner and their number decreases. This phase is the reverse of log phase.

MOLD:

  • They commonly grow as a mass of thread –like structures (hyphae) which enlarge and overlap to form visible tufts (mycelium) on food.
  • They may be growing within the food (submerged) or growing above the food (aerial).
  • Molds are non-motile, filamentous and branched organisms.
  • Their size varies from microscopic forms to macroscopic mushrooms.
  • They are multicellular organisms which lack chlorophyll and do not participate in photosynthesis.

REPRODUCTION OF MOLD:

  • They have vegetative or reproductive hyphae.
  • The reproductive hyphae usually extend in the air and form asexual exospores which are either free (conidia) in Aspergillus and Penicillium or inside a sack in.
  • The third type arthrosporic are formed when hyphae themselves get fragmented.
  • Each of these spores which are released in air for propagation is capable of forming a new mycelium.
  • It is through these asexual or sexual spores that the fungi reproduce. Black bread mold (Rhizopus) reproduces asexually by sporangiospores.
  • Sexual reproduction is less common. Downy mildews (phycomcycetes) causing blight of potatoes and buckeye rot of tomatoes can produce sexual spores known as oospores.

YEASTS:

  • They are single cell fungi,
  • Oval spherical or elongated in shape,
  • Larger than bacteria and do not form mycelium.
  • the cell wall can have scars, known as bud scars, which indicate the sites of budding.
  • They are non-motile.

REPRODUCTION OF YEASTS:

  • Yeast generally reproduce by budding an asexual process in which a bud is formed on the outer surface of the parent cell.
  • The nucleus divides and one nucleus shifts into the enlarging bud.
  • subsequently, a cell wall forms between the bud and the parent cell.
  • The bud then separated as a new cell.
  • Very few species reproduce by binary fission.

VIRUSES:

  • They are much smaller than other microbes.
  • They do not have a unicellular structure, instead, they have an outer protein layer (known as capsid) which wraps the genetic material – the nucleic acids.
  • Viruses do not have any organelles for metabolism or for independent reproduction.
  • They need access to a host to grow and multiply, accordingly, they are termed as obligate intracellular parasites.

REPRODUCTION OF VIRUS:

  • Viruses reproduce by infecting the cells of the host and exploiting their metabolism and resources for their own replication and propagation.
  • They attached themselves to the host cell and release their nuclear matter (DNA or RNA) into it.
  • The virus replicates inside the cells, subsequently the cell bursts and new viruses are released.
  • The viruses are host-specific and cannot grow or multiply outside the host.

PROTOZOANS:

  • They are the most motile unicellular organisms.
  • Most of them are free living in environment but some are parasitic and need multiple hosts to complete their life cycle.
  • At one stage they form cysts which can survive adverse environmental conditions. Generally, it is the cyst stage at which a protozoan is ingested through contaminated food.
  • The stage which causes illness is the active feeding stage (trophozoite).
  • They are more highly organized than bacteria.

REPRODUCTION OF PROTOZOANS:

  • They replicate commonly by asexual reproduction.
  • Free –living protozoans may opt for sexual reproduction in adverse environmental conditions to improve their genetics and survival through mutation and chromosomal crossing over.
  • They resort back to asexual reproduction when the food and environmental conditions become favourable.

HELMINTHS:

  • They are multicellular organisms much larger than protozoans and have complex biology.
  • The adult form for most helminths is macroscopic and is visible to unaided eye.
  • Nevertheless, they can be microscopic in size at other stages of their life cycle, e.g. egg and larval stages.

REPRODUCTION OF HELMINTHS:

  • Helminths in adult stage are macroscopic and have complex life cycles where multiple intermediate and or definitive hosts are involved and reproduction is sexual.