Microorganisms, commonly called microbes, are organisms that are too small to be seen with unaided eye. These organisms differ enormously in terms of their shape, size, appearance, nutritional requirements, metabolic capacities and genetic attributes. All these properties are used in classifying microorganisms into bacteria, fungi, protozoa, viruses, chlamydia, rickettsia and mycoplasmas. Several members of each group are capable of causing disease and so are clinically significant.
To be pharmaceutically relevant, microorganisms would not only constitute health hazard but must be destructive contaminant of pharmaceutical products. An important microbial contaminant of pharmaceutical formulation is one that has the capacity to survive (remain viable even in a dormant state) in the product, proliferate therein, metabolize some of the ingredients, or bring about spoilage of the product.
Bacteria and fungi (e.g., moulds and yeasts) are the most pharmaceutically significant organisms. Protozoa and viruses are only of pharmaceutical interest as pathogens; they are not considered as major contaminants of pharmaceutical products. This article, therefore, provides a general overview of these microorganisms
The different types of microorganisms of pharmaceutical interest
Living organisms are made up of two fundamentally different types of cells: prokaryotic cells and eukaryotic cells. Bacteria used to be considered as the only category of prokaryotic cells, but in 1990 a second group, the archaea, were recognized as having equal status to bacteria. All other organisms including humans have the eukaryotic structure with relatively more complex architecture.
Bacteria (singular: bacterium) represent the simplest living cells. They are essentially microscopic single-celled organisms, although some species arise as sheathed chains of cells. The bacteria of interest in pharmacy and medicine belong to the group known as the eubacteria (true bacteria).
The eubacteria are typically rod-shaped (bacillus), spherical (cocci), curved (vibro) or spiral (spirillum) cells of approximately 0.5 – 3.0 µm in diameter and 0.5 – 10.0 µm in length. So, when suitably stained they can easily be seen with an ordinary light microscope.
Bacteria are divided into two groups designated Gram-positive and Gram-negative cells according to their reaction to a staining procedure developed by Christian Gram in 1884. Although all bacteria that are capable of causing infection are included within this category, there are very many other bacteria that are harmless or positively beneficial.
Most bacteria of pharmaceutical interest possess cell walls and are therefore relatively resistant to osmotic stress. They grow well at temperatures between ambient and human body temperature and exhibit wide variations in their requirement for, or tolerance of, oxygen. Obligate aerobes require atmospheric oxygen for their survival and multiplication, but for obligate, or strict anaerobes oxygen is toxic.
Many other bacteria would be described as facultative anaerobes (normally growing best in air but can switch to fermentation or anaerobic respiration in the absence of oxygen) or microaerophiles (preferring oxygen concentrations lower than that in normal air). Most bacteria of pharmaceutical interest can be grown easily in the lab.
Fungi (singular: fungus) are eukaryotic organisms. Unlike bacteria, they are structurally more complex and possess distinct nuclei, each enclosed within a nuclear membrane. Fungi are normally thought of as being the toadstools and mushrooms seen on damp, rotting vegetation, but these visible parts of the fungus represent only one stage in their life cycle and other stages involve cells of microscopic dimensions; it is for this reason and the fact that many fungi never produce structures large enough to be visible to the naked eye, that they are regarded as microorganisms.
The term “fungus” covers both yeasts, many of which are only slightly larger than bacteria, and moulds of the type seen on old food in the fridge. The distinction between these two groups is not always clear. Most fungi live on dead or decaying organic materials with relatively few having pathogenic potential, but their ability to form spores that are resistant to drying makes them important as contaminants of pharmaceutical raw materials, particularly materials of vegetable origin.
Some fungi may exhibit yeast-like (unicellular) or mould-like (mycelial) appearance under the microscope depending on their growth conditions. A good example of such organism is Candida albicans which often looks like a yeast when grown in the laboratory, but exhibits a pseudomycelium and looks more like a mould when isolated from an infection site or from body fluids.
Fungi that are of pharmaceutical importance include yeasts, moulds or dermatophytes.
Yeasts are spherical or ovoid unicellular organisms that are larger than bacteria (typically 2 – 4 µm in diameter). Yeast can exhibit sexual reproduction, but more commonly they divide in the same way as bacteria either by binary fission or budding.
When growing on a Petri dish their colonies are often similar in appearance to those of bacteria, though usually larger and more frequently coloured. Very few yeasts are pathogenic; most are important as contaminants and spoilage organisms in manufactured medicines.
Mould is an imprecise term used to describe those fungi that do not produce large fruiting bodies like toadstools and mushrooms. Members of this group are structurally multicellular and grow in the form of long, slender filaments, called hyphae, which vary in width from 1 to 50 µm and may contain multiple identical nuclei.
The branching hyphae intertwine and spread over the surface of the substrate to form a mycelium. The hyphae may be septate or non-septate, but in each case the nutrients and the cellular components are freely diffusible along the filament. This is facilitated by the presence of spores in the septa (walls separating the cells).
Like yeasts, moulds are eukaryotes, but this does not mean that sexual reproduction is common. More frequently, moulds reproduce asexually and it is the formation of asexual spores that is often responsible for the characteristic colours seen in many fungal colonies. The periphery of the colony, which is the actively growing region, is often colourless.
Moulds are more significant as spoilage organisms or contaminants of manufactured medicines than as pathogens, although some are capable of causing severe illness in immune-compromised patients.
Dermatophytes are the group of aerobic fungi that can invade and infect the keratinized layers of skin, nail and hair. They are mainly the various species of Epidermophyton, Microsporum and Trichophyton.
Although dermatophytes may not produce visible deterioration of pharmaceutical products, the formulations that are contaminated by dermatophytes can become a source of infection.
Protozoa are eukaryotic, predominantly single-celled animals that are found in water and soil. The cells are typically 10 – 50 µm but can be much larger, and are usually motile. Protozoa share similar cellular characteristics with plants, animals and fungi.
Some of them feed on bacteria and can be grown in bacterial cultures in the laboratory, but most are difficult to cultivate artificially and they do not, therefore, arise as contaminants of raw materials or manufactured medicines. The great majority are harmless, but a few, such as the organisms responsible for malaria and amoebic dysentery, are capable of causing severe infection, and it is for this reason that they are of pharmaceutical interest.
Viruses are small parasites that infect all kinds of organisms: animals, plants, protozoa and bacteria too. They vary a lot in size and structure, but all contain both nucleic acid and protein; the protein surrounds and protects the nucleic acid core, which may be single-stranded or double-stranded deoxyribonucleic acid (DNA) or ribonucleic acid (RNA).
One conceptual error that is common among many beginners in the study of microbiology is their belief that viruses are microorganisms. Although they share certain attributes with microorganisms, viruses are classically, not living organisms. They are similar to microorganisms in that
- They are mostly sub-microscopic. Most viruses cannot be seen with the regular light microscope.
- They can replicate and multiply when the conditions are favourable.
- They are infectious and pathogenic, producing diseases in animal and plants.
- They possess gene derived from nucleic acid.
On the other hand, viruses differ from microorganisms in the following ways
- They are structurally smaller.
- They can survive and replicate only inside living cells (that is, they are obligate intracellular).
- Their replication is neither by binary fission nor budding.
- Each virus possesses only one type of nucleic acid that is, either DNA or RNA.
- They are not normally inhibited by antibiotics.
From a pharmaceutical point of view, viruses are not usually a threat to the physical or chemical stability of pharmaceutical products. As a contaminant of pharmaceutical formulations, they do not replicate in, or cause spoilage of products. But because their ability to cause serious diseases, pharmaceutical scientists, biochemists, molecular biologists and virologists are constantly in search of agents that can counter viral infectivity and replication.
- Denyer, S., Hodges, N., Gorman, S. and Gilmore, S. (2011). Hugo and Russell’s Pharmaceutical Microbiology. UK: Blackwell Publishing Ltd.
- Hanlon, G. and Hodges, N. (2013). Essential Microbiology for Pharmacy and Pharmaceutical Science. UK: John Wiley & Sons, Ltd.
- Okore, V. (2009). Principles of Pharmaceutical Microbiology (2nd ed.). Nigeria: Ephrata Publishers.
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