Bacteria vs Viruses vs Fungi: Key Differences

Bacteria, viruses and fungi all cause infections, and in everyday speech they get lumped together as "germs." In microbiology they are three very different kinds of things, and nearly every exam question about treatment, culture or structure depends on keeping them apart. This comparison follows the cards in Encodr's free Microbiology deck.

The comparison table

Bacteria

Bacteria are prokaryotic: no nucleus, a single circular chromosome held in a region called the nucleoid, and 70S ribosomes built from a 30S and a 50S subunit. Their cell wall is built from peptidoglycan, and the way that wall is built divides them into the gram-positive and gram-negative groups; gram-positive vs gram-negative bacteria covers it in full. They reproduce by binary fission, one cell splitting into two, which is why the bacterial growth formulas are built on doubling.

Bacteria have enormous metabolic diversity. Some are strict aerobes and some are strict anaerobes; some photosynthesize and some ferment. The course sorts them by shape (cocci, bacilli, vibrios, spirilla, spirochetes), by arrangement (pairs, chains, clusters), by wall type and by growth requirements. Archaea are also prokaryotic but have no true peptidoglycan, and no known archaeal human pathogen exists.

Viruses

A virus is not a cell. A virion is a nucleic acid genome, DNA or RNA but never both, inside a protein capsid built from capsomere subunits. Some viruses add an envelope: a lipid layer taken from the host's membrane, often studded with spikes. The four morphology classes are helical, icosahedral, enveloped and complex.

Viruses cannot replicate on their own. They need a living host cell, so they cannot be cultured on non-living media; labs use cell cultures, and for bacteriophages a soft-agar lawn of bacteria where lysis leaves clear plaques. RNA viruses must encode their own RNA-dependent RNA polymerase (RdRP), because host cells have none.

Replication follows a recognizable sequence. In the lytic cycle of a bacteriophage: attachment, penetration (only the genome enters), biosynthesis, maturation and release by lysis, which kills the host. In the lysogenic cycle, the phage genome integrates into the host chromosome as a prophage and is copied passively without killing the host, until a stressor triggers induction and a switch to the lytic cycle. Animal viruses add an uncoating step: attachment, penetration, uncoating, biosynthesis, assembly, release.

Viruses are not the only non-cellular infectious agents. Viroids are small circular RNAs with no protein coat that infect plants. Prions are infectious misfolded proteins with no nucleic acid at all, and they resist standard sterilization.

Fungi

Fungi are eukaryotes: a true nucleus, multiple linear chromosomes and membrane-bound organelles. They are non-photosynthetic. The fungal cell wall is made of chitin, not cellulose or peptidoglycan, and the fungal plasma membrane contains ergosterol, not cholesterol.

Fungi come in two basic forms. Yeasts are unicellular and reproduce by budding. Molds are multicellular, built from hyphae that form a mycelium. Dimorphic fungi grow as a yeast or a mold depending on conditions; Histoplasma capsulatum and Candida albicans are the course examples. Reproduction can be asexual (mitosis, budding, hyphal fragmentation, spore formation) or sexual (plasmogamy then karyogamy, producing zygospores, ascospores or basidiospores).

Fungal infections are called mycoses. Named pathogens include Aspergillus, the dermatophytes (Trichophyton, Microsporum and Epidermophyton, which infect keratinized tissue and cause tinea or ringworm), Blastomyces dermatitidis, Coccidioides immitis and Cryptococcus neoformans.

Why treatment differs

This is the payoff of learning the differences, and it is the question exams most like to ask. A good drug shows selective toxicity: it harms the pathogen while sparing host cells. The way to get that is to attack something the pathogen has and the host lacks.

Antibiotics work on bacteria because bacteria have peptidoglycan and 70S ribosomes. They do nothing against a virus, which has neither. Antifungals target ergosterol, which fungal membranes have in place of the host's cholesterol, or the fungal wall.

The course also explains why there are fewer drugs for fungi, protozoa and helminths than for bacteria. Those organisms are eukaryotic like the host, which makes a selective target harder to find. Viruses are hard for a different reason: they hijack host machinery.

A quick way to sort an exam question

1. Does the question mention a cell wall? Peptidoglycan means bacteria and chitin means fungus. A protein capsid means a virus.
2. Does it mention a nucleus, ergosterol or hyphae? That is a fungus.
3. Does it mention needing a host cell, a capsid or a prophage? That is a virus.
4. Does it ask what a drug targets? Match the target to the organism using the table above.

Studying the differences

Comparison tables are the best format for this topic, because the facts only make sense side by side. Build the table above from memory, then check it, and repeat a few days later: active recall works better than rereading. For the full list of topics, see what's in microbiology, and for a unit-by-unit plan see how to study for microbiology.