What is the difference between a bacterium and a protozoan?
Question
The other day while revising for my microbiology exam, I came across an interesting difference between bacteria and protozoa. I had always thought they were both just tiny microbes, but turns out they are structured very differently. One is prokaryotic and the other eukaryotic — and that changes everything from how they feed to how they reproduce.
Answer ( 1 )
Cellular Organization
The most fundamental distinction lies in cellular organization. Bacteria are prokaryotic organisms, characterized by cells lacking membrane-bound organelles, including a true nucleus. Their genetic material (typically a single, circular chromosome) floats freely in the cytoplasm in a region called the nucleoid. In contrast, protozoa are eukaryotic microorganisms with complex cellular organization, including membrane-bound organelles such as a true nucleus (containing multiple linear chromosomes), mitochondria, endoplasmic reticulum, Golgi apparatus, and in some cases, specialized structures like hydrogenosomes or chloroplasts.
Size
Bacteria are typically much smaller than protozoa, ranging from 0.2 to 2.0 micrometers in diameter for most species (though some exceptional bacteria like Thiomargarita namibiensis can reach 750 micrometers). Protozoa are generally much larger, typically ranging from 10 to 100 micrometers, with some species reaching several millimeters in length.
Metabolic Capabilities
Bacteria display extraordinary metabolic diversity:
They can utilize an extraordinary range of energy sources, including hydrogen, sulfur, iron, and ammonia. Many bacteria thrive in extreme conditions. Protozoa are primarily heterotrophic, obtaining nutrients by ingesting bacteria, organic particles, or host tissues. Some like Euglena can perform photosynthesis or form symbiotic relationships with photosynthetic microbes.
Feeding Mechanisms
Bacteria absorb nutrients across their membrane after releasing enzymes externally. Protozoa use complex strategies like:
Some parasitic protozoa extract nutrients directly from host tissues.
Locomotion
Bacteria move using:
Protozoa use:
Reproduction
Bacteria reproduce asexually via binary fission. Genetic material can also be exchanged through conjugation, transformation, or transduction, though this isn’t true sexual reproduction. Protozoa reproduce both asexually (binary/multiple fission, budding) and sexually (via meiosis and fusion of gametes), enabling greater genetic diversity.
Ecological Roles
Bacteria play a major role in nutrient cycling (e.g., nitrogen fixation, decomposition) and form the base of many food webs. Protozoa are key bacterial predators and nutrient recyclers in aquatic and soil ecosystems. They are also food for larger organisms.
Medical Significance
Bacterial infections are common and include pneumonia, meningitis, and sepsis. These are usually treated with antibiotics targeting bacterial cell structures. Protozoan infections are less common but serious, including:
They require antiprotozoal treatments targeting unique eukaryotic features.
Evolutionary History and Diversity
Bacteria are ancient and highly diverse, dating back over 3.5 billion years. They are divided into numerous phyla based on genetic data. Protozoa are not a single taxonomic group but belong to various lineages like Amoebozoa, Excavata, Alveolata, and Rhizaria.
Understanding these differences is essential for diagnosing infections, managing microbial growth, and appreciating microbial community interactions in both natural and engineered environments.
Source: Brock Biology of Microorganisms; Protist Diversity and Geographical Distribution (book)