Bacteria are microscopic unicellular organisms found almost everywhere on the Earth. These are critical for ecosystems as they effortlessly recycle nitrogen, carbon, and sulfur. These can also cause harmful diseases to humans, animals, and plants. Some bacterial species can grow in extreme conditions like high temperature, low pH, and high pressure. 

Fungi are microscopic unicellular or multicellular organisms. Most fungi inhabit soil and dead organisms and play a crucial role in mineralizing organic carbon. Fungi are opportunistic pathogens. But mainly, these decay and decompose food, dead animals, and plants.

The significant difference between bacteria and fungi is their cell; bacteria are prokaryotes, and fungi are eukaryotes. In addition, bacteria and fungi have differences in their growth requirements, cell wall components, size, morphology, colony characteristics, and treatment methods.  

Although bacteria (Monera) and fungi (Mycota) have many differences, they have similarities as well. Both have true cell walls and many beneficial properties, and some species of both kingdoms can cause serious diseases.

Similarities Between Bacteria and Fungi

• Both are microscopic organisms.
• Fungi and bacteria have a cell wall made up of polysaccharides. Although the components of the cell walls are different, all species of bacteria and fungi have a true cell wall.
• Some bacteria like Lactobacillus are used as probiotics, Staphylococcus epidermidis as normal skin flora that protects skin from infection, and Escherichia coli in the gut benefit humans. Likewise, yeast is helpful in the production of alcohol and bakery items.
• Some species of both phyla can cause serious illness. Like aspergilloma (fungus ball) by Aspergillus species, a fungus that can seriously damage the lungs of an infected human. Likewise, cholera by Vibrio cholerae and typhoid by Salmonella species are some of the life-threatening water-borne bacterial diseases.
• Both of the organisms can be saprophytic or parasitic. E coli and Spirochetes are saprophytic bacteria, whereas almost all fungi are saprophytic.
• Both bacteria and fungi are heterotrophs, meaning they can grow and feed on plants and animals.
• Both bacteria and fungi require heat, nutrition, and moisture for growth.
• The optimum temperature for bacteria and fungi that can cause human infections is 37℃

Difference Between Bacteria and Fungi

References

• Madiga, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed., p. 601). Pearson Education.
• Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th ed., pp. 334). Tata McGraw-Hill.

Fungi are a group of eukaryotic spore-bearing fungi lacking chlorophyll. The fungi require organic compounds for nutrition. They are heterotrophic organisms that require organic compounds for food. Mycology is the study of fungi. Fungi are of two types; yeast and molds.   

A mold is a multicellular fungus growing in filaments called hyphae. The hyphae are multicellular and have many genetically identical nuclei, but it forms a single organism. However, yeast is a unicellular fungus; that is, the thallus (body of fungi) comprises of a single cell. 

Although yeast and mold have many differences, they possess similarities. One being the phylum and another being their cell wall has chitin. 

The Difference Between Yeast and Mold

Another type of fungi in yeast, as well as mold, forms are the dimorphic fungi. It converts into the mold when the temperature is 20-25℃ and into yeast at 37℃. It is responsible for causing many diseases in humans and animals. Examples of dimorphic fungi are Histoplasma capsulatum, Blastomyces dermatitidis, Coccidiodes immitis, etc.

References and detailed study

Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th ed., pp. 333-362). Tata McGraw-Hill.

Viruses are submicroscopic particles made by complicated assemblies of proteins, nucleic acids, lipids, and carbohydrates. These smallest infectious agents can infect a human, animal, plant, or bacterial cells. Viruses are strict obligate intracellular parasites incapable of replication without living hosts. Due to the lack of independent replication capability, viruses are considered non-living. 

Bacteria are unicellular prokaryotes. They are ubiquitous and play both beneficial and harmful roles.  Bacteria are used as micro-factories in the industries for synthesizing various important compounds, as a starter culture for making yogurt and cheese, and as probiotics to prevent infections. Pathogenic bacteria can cause diseases from small boils to deadly diseases like meningitis or plague. 

There are many clear-cut structural differences between bacteria and viruses such as size, cellular components, nature, and size of nucleic acid. Other differences lie in the method of replication, host cell requirements, and weapons used to kill them. The main question which always intrigued many is, are viruses alive?

Some of the major differences between bacteria and viruses are summarized in the table below;

* 1 Mb = 1,000,000 bases

You can visit this blog if you are interested in knowing the difference between bacterial and viral infections.

 References and further readings 

1. Brown N & Bhella D (2016). Are Viruses Alive, Issue: What is Life? Microbiology Society.
2. What’s the difference between bacteria and viruses? The University of Queensland Australia. Retrieved on 5th July, 2022 from https://imb.uq.edu.au/article/2020/04/difference-between-bacteria-and-viruses 
3. Claverie JM, Grzela R, Lartigue A, Bernadac A, Nitsche S, Vacelet J, Ogata H, Abergel C. Mimivirus and Mimiviridae: giant viruses with an increasing number of potential hosts, including corals and sponges. J Invertebr Pathol. 2009 Jul;101(3):172-80. doi: 10.1016/j.jip.2009.03.011. Epub 2009 May 18. PMID: 19457438.
4. Volland JM, Gonzalez-Rizzo S, Gros O, Tyml T, Ivanova N, Schulz F, Goudeau D, Elisabeth NH, Nath N, Udwary D, Malmstrom RR, Guidi-Rontani C, Bolte-Kluge S, Davies KM, Jean MR, Mansot JL, Mouncey NJ, Angert ER, Woyke T, Date SV. A centimeter-long bacterium with DNA contained in metabolically active, membrane-bound organelles. Science. 2022 Jun 24;376(6600):1453-1458. doi: 10.1126/science.abb3634. Epub 2022 Jun 23. PMID: 35737788.

Bacterial infections are caused by bacteria, whereas viruses cause viral infections. Whatever the etiological agent, an infection in a person typically has four stages: 

1. incubation period: a period when the patient is asymptomatic, 
2. prodromal period: a period of non-specific symptoms, 
3. specific-illness period: a period when characteristics, symptoms, and signs appear, and 
4. recovery period: in this period, illness wane, and the patient regains good health.

While bacteria and viruses can both cause mild to serious infections, use similar modes of transmission, and have almost similar courses of infection, these infections also differ. The easiest difference anyone can figure out is bacterial infections are caused by bacteria, while viruses cause viral infections. Perhaps the most applicable difference is antibiotics are used to kill bacteria but ineffective for viruses.

Features of Viral Infections

1. Most viral infections are asymptomatic. Of those infections that are symptomatic, most are mild and self-limiting. Most viral infections are either localized to the portal of entry (common cold involves only the upper respiratory tract, influenza is localized in upper and lower respiratory tracts) or spread systemically throughout the body, for example, poliomyelitis, measles, etc. Systemic infections are the most serious viral infections.
2. The severity and outcome of viral infections are affected by several variables such as age, immune status, nutritional status, etc.
   1. Age is a significant variable in the outcome of viral infections. Generally, infections are more severe in neonates and the elderly than in older children and young adults.
   2. Malnutrition leads to more severe viral infections. For example, cases of measles infections in developing countries.
   3. Immunosuppression predisposes to various viral infections and increases the severity of the disease, which is mild in the immune-competent hosts.
   4. Increased corticosteroids suppress host defenses and predispose them to more severe viral infections, such as varicella-zoster virus, disseminated herpes virus infection, etc.
3. Types of viral infections
   • Disseminated or systemic viral infections: Examples include measles, mumps, rubella, and chickenpox
   • Local viral infection: For example, the common cold is a viral infection of the upper respiratory tract.
4. Though viruses are intracellular obligate parasites, both humoral immunity and cell-mediated immunity play active roles in controlling viral infection. Cytotoxic T cells kill the virally infected cells, and antibodies neutralize the viruses circulating in the body fluids such as blood and CSF. 
5. Viral infections are often diagnosed by PCR assay for viral DNA or RNA or by serologic tests for specific antibodies or detection of specific viral antigens.

You may also like to read: Differences Between Bacteria and Viruses

Features of Bacterial Infections

1. The initial step in the process of many bacterial infections is the adherence of the organism to mucous membranes. Many bacterial components such as pili (fimbriae), glycocalyx, capsule, lipoteichoic acid, etc. mediate the attachment process.
2. Most bacterial infections are acquired from external sources. However, some bacterial infections are caused by members of the normal flora when there are breaches in the anatomical barriers (e.g. trauma) or when the host immune status is weakened.
3. Most bacterial infections are communicable but some are not, for example, botulism and Legionella pneumophila.
4. Bacteria cause disease by two major mechanisms: toxin production (exotoxins and endotoxins) and invasion and inflammation.
5. Polymorphonuclear leukocytes (PMNs) are the predominant cellular defense in bacterial infection.
6. Host defenses against bacterial infections include both innate and adaptive immunity. Innate defenses are nonspecific and may include physical barriers, cells, and proteins. Adaptive (acquired) defenses are highly specific for the organism and include antibodies and cells such as CD4-positive helpers;
7. Microbiologic diagnosis of bacterial infections frequently is made using Gram stain and culture. Gram stain evaluation of direct smears is routinely used to diagnose bacterial infections.
8. Serologic tests and polymerase chain reactions (PCR) assays are also useful for diagnosing viral infections. Serologic tests are done for systemic bacterial infections, whereas PCR assays are reserved for non-cultivable or hard-to-culture bacterial pathogens.

Differences between Bacterial and Viral Infections 

References

• Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed). Pearson Education
• Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition
• Review of Medical Microbiology and Immunology, Lange Medical Books, 13th edition