Microorganism

Microorganisms are microscopic, invisible to the naked eye, and mostly unicellular organisms. Bacteria, viruses, fungi, protozoa, and some algae belong to this group. As the oldest and most widespread forms of life, microorganisms can live in soil, water, air, and within or on living organisms. While some can harm their host, the majority play symbiotic or beneficial roles. Microbial life is an integral part of Earth’s biosphere and holds a central role in ecosystem functioning.

Classification and Major Groups

Microorganisms are generally classified into prokaryotes (bacteria and archaea) and eukaryotes (protists, some algae, fungi). Prokaryotes lack a nucleus, with their genetic material freely suspended in the cytoplasm. Eukaryotes possess a membrane-bound nucleus and organelles. Viruses are classified separately because they cannot replicate outside a living cell. Microorganisms are also subdivided based on Gram staining, metabolic characteristics, cell wall structure, and oxygen requirements.

Growth Conditions and Factors

Microorganism growth depends on internal and external factors such as pH, water activity, temperature, nutrients, and gas composition. For example, bacteria typically grow within a pH range of 4.5–9.0, while molds can survive even at very low pH values such as 1.5. Water activity (Aw) is critical for microbial cellular activities. Most pathogenic bacteria cannot grow below an Aw of 0.91, whereas some mold species can multiply at an Aw of 0.80. Temperature is also a key factor; some bacteria grow optimally at 37°C, while extremophiles can survive at extremely low or high temperatures.

Growth Phases and Cellular Processes

The reproduction of microorganisms is described by a growth curve consisting of distinct phases: lag (adaptation) phase, log (exponential growth) phase, stationary phase, and death phase. During the lag phase, no cell division occurs, but cells adapt to the environment. In the log phase, cells multiply rapidly, and the specific growth rate reaches its maximum. In the stationary phase, growth halts due to nutrient depletion and accumulation of toxic waste products. In the death phase, cell death exceeds the rate of reproduction, leading to a decline in population. These processes are critically important in industrial microbiology for enhancing production efficiency.

Role in the Human Body and the Microbiota

The human body harbors more microorganisms than human cells. This community of microorganisms is called the microbiota and is most densely populated in the gastrointestinal tract. Different microbial densities and species exist along the stomach, small intestine, and large intestine. For instance, gram-positive anaerobic bacteria (Bacteroides, Clostridium, Bifidobacterium) dominate the colon. These microbial communities perform multifunctional roles, including immune system development, inhibition of pathogens, synthesis of certain vitamins (K, B12), and support of digestion.

Clinical Significance of Microorganisms

Hospital-acquired infections pose a serious problem, particularly in intensive care units. Among microorganisms isolated from clinical samples, the most common are Pseudomonas spp., Escherichia coli, Acinetobacter spp., and Klebsiella spp. The antibiotic resistance profiles of these microorganisms directly affect treatment success. Strains such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus spp., and carbapenem-resistant Enterobacteriaceae cause severe nosocomial infections. Therefore, antimicrobial susceptibility testing is vital for establishing empirical treatment protocols.

Probiotics and Their Relationship to Health

Probiotics are live microorganisms that confer a beneficial effect on the host’s intestinal flora. Genera such as Lactobacillus and Bifidobacterium are used to prevent gastrointestinal infections, modulate the immune system, and reduce inflammatory diseases. Probiotic bacteria produce antimicrobial substances such as bacteriocins, hydrogen peroxide, and organic acids; they defend against pathogens through competitive adhesion; and they modulate immune responses via regulatory T cells. Additionally, research is ongoing into the potential of certain probiotic strains to reduce allergic reactions.