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.
