Microorganisms inhabit soil, water, and the surfaces and interiors of living organisms. Bacteria and fungi are two of the most commonly encountered groups. While often grouped together as “germs,” they represent fundamentally distinct forms of life belonging to separate biological kingdoms. Understanding these differences dictates how they function, how they grow, and how humans effectively combat them when they cause illness.
The Core Biological Distinction
The fundamental difference between fungi and bacteria lies in their basic cellular organization. Bacteria are classified as prokaryotes, possessing a simple cellular architecture that lacks internal compartmentalization. Bacterial cells do not possess a membrane-bound nucleus or specialized, enclosed structures like mitochondria.
Fungi are eukaryotes, placing them in the same biological domain as plants, animals, and humans. Eukaryotic cells have a complex internal structure, including a true nucleus that organizes the cell’s DNA. They also contain various membrane-bound organelles, such as mitochondria, which manage energy production. This difference means that a fungal cell is significantly more complex than a bacterial cell.
Physical and Chemical Composition
The external physical and chemical makeup of these two microbial groups shows significant differences. The protective cell wall surrounding a bacterium is primarily constructed from a complex polymer called peptidoglycan. This material provides structural integrity and is unique to almost all bacteria, making it a target for certain therapies.
The cell wall of a fungus is built mostly from chitin, the same tough polysaccharide found in the exoskeletons of insects and crustaceans. Fungi are typically larger than bacteria, with bacterial cells ranging from 0.5 to 5 micrometers, whereas fungal cells are often 2 to 10 micrometers in size. While bacteria exist mainly as single cells, fungi can be single-celled (yeasts) or form complex, thread-like, multicellular structures known as hyphae.
Fungi and bacteria also employ different strategies for obtaining energy and nutrients. All fungi are heterotrophs, meaning they must absorb pre-existing organic compounds from their environment, such as decaying matter or a living host. Bacteria exhibit a greater metabolic diversity, with some species being heterotrophs while others are autotrophs capable of producing their own food using light or inorganic chemicals. This variety allows bacteria to colonize a much broader range of environments.
How Differences Affect Treatment
The structural and biological differences between the two groups have consequences for treating the infections they cause. The goal of antimicrobial treatment is selective toxicity: the drug must harm the pathogen without damaging the patient’s own cells. This is relatively straightforward with bacteria due to their prokaryotic nature and unique features like the peptidoglycan cell wall.
Antibiotics exploit these differences, often by inhibiting the synthesis of peptidoglycan, which causes the bacterial cell to burst and die. They can also target the smaller 70S ribosomes found in bacteria, preventing protein production without affecting the host’s 80S ribosomes. Since fungi are eukaryotes and share many cellular processes and structures with human hosts, finding targets for treatment is harder.
Therapies against fungi, known as antifungals, must focus on the fewer differences that exist to achieve selective toxicity. The fungal cell membrane uses a sterol called ergosterol for stability, while human cells use cholesterol. Antifungal medications work by binding to or inhibiting ergosterol production, which disrupts the fungal cell membrane and leads to the death of the organism. Antifungal drugs are generally fewer in number and can sometimes have more side effects than antibacterial agents.