INTRODUCTION TO MICROBIOLOGY
Microorganisms are the dominant life forms on earth. They are found in
almost every conceivable environment and are essential to sustaining
life on earth. Basically there are five groups of microorganisms,
bacteria, fungi, viruses, protozoa and algae.
Microorganisms are also called microbes, they can be found in virtually
all ecosystems and they exist in very close association with every type of
multicellular organism. Microorganisms have an extraordinary
diversity, in structure, function, habitat and applications. They are
found in every corner of the globe, they are indispensable to life on
earth.
Microbiology is the study of microorganisms. It is defined as the science
(logos), of small (micro) life (bios). Microbiology has been defined as the
study of living things so small that they cannot be seen with the naked
eye. However, this definition does not fully conceptualize the various
organisms studied in microbiology. This is because bacteria neatly fit
this definition. However, fungi and algae contain members that are not
microscopic. Also, certain organisms such as nematodes can be
microscopic but are not considered to be the domain of microbiology.
Viruses represent another special case. They are submicroscopic but are
not living organisms but they fall within the domain of microbiology.
Microorganisms have a tremendous impact on all life and the physical
and chemical makeup of our plants. They are responsible for recycling
the chemical elements essential for life such as carbon, nitrogen, sulfur,
hydrogen and oxygen. More photosynthesis is carried out by
microorganisms than by green plants. Humans have an intimate
relationship with microorganisms. More than 90% of the cells in our
bodies are microbes.
Microorganisms are the products of evolution, the biologic consequence
of natural selection, operating on a vast array of genetically diverse
organisms. Microorganisms are highly complex and diverse, they are
the most heterogenous subset of all living creatures and comprise of
prokaryotes, eukaryotes and accelular particles like viruses and prions.
Bacteria are the prokaryotes whereas among the groups of eukaryotes
are the protozoa, fungi, slime molds and algae.
All prokaryotic organisms are classified as bacteria while eukaryotic
organisms include fungi, protozoa and helminthes. Prokaryotic
organisms are broadly divided into two groups; eubacteria and
archeabacteria. The eubacteria include all bacteria of medical
importance while the archeabacteria are a collection of evolutionary
distinct organisms. Prokaryotic cells are smaller than eukaryotic cells.
Generally, bacteria can broadly be divided into two; typical bacteria and
atypical bacteria. Most typical bacteria have shapes that can be
described as either a sphere (cocci), a rod (bacilli), or corkscrew (helical).
Nearly all bacteria have a rigid cell wall surrounding the cell membrane
that determines the shape of the organism. Bacterial cells divide by
binary fission. They may or may not possess flagella, pilli and capsule.
Also, many bacteria exchange genetic information among themselves
through various strategies. This is called horizontal gene transfer.
Atypical bacteria are a group of bacteria that lack significant
characteristic structural components or metabolic capabilities that
separate them from the larger group of typical bacteria. Some atypical
bacteria are the Mycoplasma, Chlamydia and Rickettsia.
Fungi are eukaryotic organisms which are saprophytic and
nonphotosynthetic. Some fungi are filamentous and are called molds
while others are unicellular and are called yeasts. Some are dimorphic
and can exist both as yeasts and as molds depending on the
atmospheric conditions of their growth. Fungi reproduce by either
sexual or asexual reproduction or both. All fungi produce spores. While
most fungi are beneficial to man, some of them may be pathogenic and
can cause various diseases.
Protozoa are unicellular, nonphotosynthetic eukaryotic organisms that
exist in diverse shapes and sizes. Many protozoa are free-living but
others are among the most clinically important parasites of humans.
They can be intracellular parasites in the blood or tissues
or
extracellular parasites on the skin.
Helminthes are a group of worms that live as parasites. They are
multicellular, eukaryotic organisms with complex organization.
Helminthes are classified into cestodes (tapeworms), trematodes
(flukes), and nematodes (roundworms). Helminthes are mostly parasitic
though some are free living. Parasitic helminthes obtain their nutrients
by ingesting or absorbing digested nutrients or ingesting body fluids or
tissues. They can parasitize almost any organ in the body.
Viruses are acellular structures. They are obligate intracellular
parasites that do not have a cellular structure. A virus consists of a
genetic element (either a DNA or RNA but not both) surrounded by a
protein coat. A virus may also have an envelope derived from the
plasma membrane of the host cell from which the virus is released.
Viruses contain the genetic information necessary for directing their
own replication but require the host’s cellular structures and enzymatic
machinery to complete the process of their own reproduction. The fate of
the host cell following viral infection ranges from rapid lysis releasing
many progeny virions to gradual prolonged release of viral particles.
Viruses cause a broad spectrum of diseases.
Prions are designated as infectious proteins. They are relatively
resistant to proteolytic degradation and when infectious, they tend to
form insoluble aggregates of fibrils. Prions are unconventional
infectious agents. Prions are distinctive transmissible infectious agents
that although having some properties in common with viruses do not fit
the classic definition of a virus. Prions have been implicated as the
causative agents of transmissible spongiform encephalopathies (TSE).
TSEs occurring in humans manifests as Kuru, Creutzfeldt-Jakob
disease (CJD), and fatal familial insomnia (FFI). These diseases are
characterized by spongiform vacuolation of neuronal processes and grey
matter, accumulation of a unique protein called prion protein in the
brain.
The discipline of microbiology now includes microbial molecular biology
and functional ecology as well as the traditional studies of structure and
physiology. The discipline began in the late 17th century with Antonio
Von Leeuwenhoek’s discovery of bacteria using a simple microscope of
mixed natural cultures. Through the 1850s and 1860s, Louis Pasteur’s
simple experiments using sterilized beef broth finally refuted the long
held theory of spontaneous generation as an origin for microbes and
microbiology moved into mainstream science.
The early days were characterized by studying environments like soil
and sediments, natural fermentations and infections and it was not
until Robert Koch developed techniques for pure culture in the late 19 th
century that the science moved to a reductionist phase where microbes
were isolated and characterized in the laboratory.
Through the 20th century, microbiologists focused on the discovery and
characterization of many different microorganisms including a new
kingdom of microorganisms (the Archaea), new eubacterial pathogens
like Legionella and Methicillin-resistant Staphylococcus aureus (MRSA)
and the complex of pathogens associated with HIV including the fungal
pathogen Pneumocystis. The discovery of the unique communities found
in extreme environments with their temperature-tolerant DNA
polymerase enzymes has further opened up the new field of molecular
biology.
The rapid advances of techniques in molecular biology has allowed
microbiology to return to the natural environments. Techniques such as
denaturing gradient gel electrophoresis, single-stranded conformation
polymorphism, DNA chips and in-situ hybridization now give us the
tools to study microbial ecology at the molecular level. This has
returned microbiology to its roots.