7.5: Medical Applications of Microbial Biotechnology

Biotech Focus

In 1980, the World Health Organization (WHO) declared the disease smallpox officially eradicated. Smallpox is a highly contagious, deadly disease caused by the variola virus. Infected individuals become covered with pus-filled lesions, with the majority found around the head (Figure \(\PageIndex{1}\)). Smallpox has no known cure, killing 1 in 3 infected people. Those surviving are left with deep scars across their body, with some people blinded by the disease. The first evidence of smallpox were found in Egyptian mummies dating back from 1350 BCE, whose skin was covered with lesions that resembled those of the smallpox rash. To combat this disease, early physicians in Asia and Africa used the practice of variolation, which consisted of transferring pus collected from smallpox sores to healthy individuals to induce a milder illness. Although variolation was mostly successful, it presented many risks, such as introducing an opportunistic infection. In the 18th century, Edward Jenner created the first smallpox vaccine using the cowpox virus, a closely relative to smallpox. Jenner collected pus from a cowpox blister from the hand of an infected milkmaid and use it to inoculate a cut in a boy's arm. When exposed to smallpox, the boy did not develop the disease. Subsequent advances in research and production has resulted in a safer and more stable vaccine. The current smallpox vaccine is made from a "live" virus related to smallpox. It does not contain the virus and cannot cause smallpox but provides immunity.

Figure \(\PageIndex{1}\): Left image: Smallpox lesions. (Obsolescent variolous lesions by Wellcome Collection Gallery, CC BY-SA 4.0) Right image: An unvaccinated boy surrounded by his two siblings who were vaccinated for smallpox. Note the absence of lesions in the unvaccinated siblings. (Smallpox in siblings by National Museum of Health and Medicine, CC BY 2.0).

Introduction

For centuries, natural products have been the cornerstones of traditional medicine. Today, microbial biotechnology is revolutionizing medicine by changing how we approach drug discovery and treatment, and offering new solutions to combat diseases, like infections and cancer. Microbes, like bacteria and fungi, are proving to be invaluable in the pharmaceutical industry. Bacteria and fungi are being used as a rich source of bioactive compounds with therapeutic potential. Bioactive compounds are naturally occurring chemicals found in plants, animals, microbes, or other biological sources, that have an effect on a living organism. These compounds include antibiotics, anti-fungals, and anti-cancer agents. Genetic engineering of these microbes, together with synthetic biology are being used to enhance microbial production of these compounds, discover new drug leads, and develop innovative therapies such as vaccines and targeted drug delivery systems.

Learning Objectives

Microbial biotechnology is a key part of the pharmaceutical industry, revolutionizing the development of drugs and the treatment of diseases. At the end of this page, you will be able to:

• Define key microbial biotechnology terms in relations to medicine
• Describe the three basic types of antibiotics
• Describe how microbial biotechnology is used in the production of antibiotics
• Describe the major types of vaccines that use microbial technology in their production

Microbes as drug factories

Antibiotic Production

An antibiotic is a type of antimicrobial agent specifically designed to fight bacterial infections by either killing the bacteria (i.e., bactericidal antibiotics) or by inhibiting their growth (i.e., bacteriostatic antibiotics). Antibiotics are considered to be secondary metabolites - compounds not essential for the growth of the organism. Many antibiotics, such as penicillin from Penicillium fungi or streptomycin from Streptomyces bacteria, are naturally synthesized by microbes to inhibit the growth of bacteria. These antibiotics are called natural antibiotics. In addition, there are two other types of antibiotics: semisynthetic, and synthetic (Figure \(\PageIndex{2}\)). Semi-synthetic antibiotics are modified versions of natural antibiotics, with chemical alterations to enhance their effectiveness, stability, or resistance to bacterial degradation. One well-known example is amoxicillin, an antibiotic derived from penicillin G, and enzymatically modified to remove a side chain from the natural version. Synthetic antibiotics, on the other hand, are artificially designed and chemically synthesized compounds that are designed to target critical bacterial processes. Synthetic antibiotics are man-made and are often based on knowledge obtained from natural antibiotics. Synthetic antibiotics include novel compounds, such as the fluoroquinolones, which are engineered to inhibit bacterial DNA replication.

Figure \(\PageIndex{2}\): Types of antibiotics. Antibiotics are classified as natural, semi-synthetic, and synthetic. Natural antibiotics are compounds produced by microbes and include agents such as penicillins G/V, streptomycin, and cephalosporin C. Semi-synthetic antibiotics are chemically-modified natural antibiotics and include compounds like amoxicillin and cephalexin. Synthetic antibiotics are industrially manufactured agents chemically related to natural antibiotics. Synthetic antibiotics include the fluoroquinolone and sulfonamide families of agents. (Types of Antibiotics by Patricia Zuk, CC BY 4.0)

The production of natural and semi-synthetic antibiotics at an industrial scale, together with many synthetic versions, involves a fermentation step at some point in their production (Figure \(\PageIndex{3}\)). In industrial fermentation, the microbe is grown in large bioreactors (100,000 – 150,000 liters or more), called fermenters, containing a liquid growth medium. The size of the microbe population must be controlled very carefully to ensure that maximum yield of the antibiotic is obtained before the cells die. Therefore, oxygen concentration, temperature, pH, and nutrient levels are optimized, closely monitored, and adjusted if necessary. For more information about fermenters, go to Chapter 7.3 Microbial Cultivation and Manipulation. Once the fermentation process is complete, the antibiotic is extracted and purified. For semisynthetic and synthetic antibiotics, continued chemical processing is required.

Figure \(\PageIndex{2}\): Natural and synthetic antibiotic production. The steps involved in the production of the natural antibiotic, penicillin G, and the semisynthetic version, amoxicillin are shown in this figure, together with the chemical structures of the critical compounds. Both antibiotics begin with large-scale fermentation of the fungus, Penicillium chrysogenum under controlled conditions in a bioreactor. Penicillin G is extracted from the bioreactor. In amoxicillin production, penicillin is enzymatically converted to 6-APA, which is then chemically modified into amoxicillin. At the end of this process, both penicillin G and amoxicillin are formulated into medicines. (Antibiotic production by Patricia Zuk, CC BY 4.0; figure created in BioRender. Zuk, P. (2025))

Despite their variety, approximately 99% of antimicrobial agents, such as antibiotics, have little medical or commercial value. Many of these "low-value" antibiotics simply lack a significant advantage over those already in use or have no practical application. This, together with the rise of antibiotic resistance, requires continued exploration and optimization of antibiotic development and production. To promote this, pharmaceutical companies use microbial biotechnology to enhance antibiotic production, develop new antibiotics, or modify existing ones to make them more efficacious. The introduction of mutations (i.e. mutagenesis) into the microbial genome to boost antibiotic production is a common microbial biotechnology method. Mutagenesis is produced by exposing microbial cells to mutagens such as ultraviolet radiation, x-rays, or certain chemicals. Selection of microbial strains for high-yield antibiotic production is followed by propagation in bioreactors and purification (see Figure \(\PageIndex{2}\)). Mutated strains with antibiotic yields as high as 20-fold or more have been produced using this technique.

Biotechnologists can also boost antibiotic production through gene amplification, where copies of genes coding for enzymes involved in antibiotic production can be inserted back into the microbial cell, via vectors such as plasmids. Genetic engineering can create new antibiotics by modifying microbial pathways. Altering microbial DNA can be used to produce entirely new synthetic antibiotics. Genes from different bacteria can be combined to generate novel antibiotic structures. Gene editing through techniques like CRISPR are also being explored.

Vaccine Production

A vaccine is a biological preparation that stimulates the host's immune system to recognize and fight a specific pathogen (i.e., a virus, bacterium, or other microbe) without causing the actual disease. Vaccines help the body develop immunity by preventing future infections or reducing their severity. There are a wide variety of vaccines technologies used today, with some technologies produced for a specific pathogen (Table \(\PageIndex{1}\)). The two most common types of vaccines used today are the live-attenuated vaccine, which uses a weakened version of a microbe, and the inactivated (killed) vaccine, which uses a killed version. These two vaccines are relatively inexpensive to produce and generate a robust immune response.

Table \(\PageIndex{1}\): Vaccine Types

Vaccine Type	Antigen used	Pathogen	First introduced
Live attenuated (weakened of inactivated)	uses a weakened (attenuated) form of a virus or bacterium the pathogen is modified so that it cannot cause disease in healthy individuals but still stimulates a strong and long-lasting immune response.	measles mumps rubella yellow fever influenza varicella (chicken pox) typhoid	1798 (smallpox)
Killed	uses a killed virus or bacterium	pertussis polio influenza hepatitis A rabies	1896 (typhoid)
Toxoid	uses an inactivated toxin produced by bacteria the toxin is chemically treated to render it harmless but is still capable of stimulating an immune response the immune response doesn't target the bacteria but creates antibodies that neutralize the toxin during an infection	tetanus diptheria	1923 (diptheria)
Subunit	made from a fragment, like sugars and proteins, isolated from a pathogen the fragments can be produced in large quantities by genetically-engineered bacteria (e.g., E.coli) or yeast (e.g., Saccharomyces cerevisiae, Pichia pastoris)	hepatitis A & B influenza pertussis meningococcal pneumococcal	1970 (anthrax)
Viral vector	uses harmless viruses or bacteria (i.e., the vector) to deliver genetic material into cells the bacteria or virus is genetically modified to deliver pathogenic genes into host cell protein production by host cells generates a strong immune responses viral vector vaccines currently in use; bacteria vector vaccines are experimental	Ebola	2019 (Ebola)
Outer membrane vesicle (OMV)	uses membrane fragments, called vesicles, that are released from the outer membrane of certain gram-negative bacteria the vesicles contain various components from the bacterium's outer membrane which are crucial for the immune reaction	Group B meningococcal	1987 (meningococcal group B)
Protein-polysaccharide conjugate	uses sugars from the outer capsule of a bacteria the sugar is coupled (conjugated) to a viral protein to boost the immune response	Hemophilus influenzae type B pneumococcal meningococcal typhoid	1987 (Hemophilus influenzae type B)

In addition to these two types of vaccines, there are additional types of vaccines that use microbial biotechnology for their development. These more advanced vaccine technologies are capable of decreasing the cost of vaccine production and increasing production efficiency through their use of microbes to produce the antigens and adjuvants that are key to vaccine production, in addition to using microbes that act as vectors to stimulate the immune response.

Key Concepts

Microbial biotechnology is revolutionizing medicine by changing drug discovery and treatment, and offering new solutions to combat diseases, like infections and cancer.

Some important concepts to remember are:

• a bioactive compound is a naturally occurring chemical found in plants, animals, microbes, or other biological sources, that is used to affect an organism
• an antibiotic is a compound that is used to treat bacterial infections
• a natural antibiotic is naturally synthesized by microbes to inhibit the growth of bacteria
• a semisynthetic antibiotic is produced by chemically modifying a natural antibiotic
• a synthetic antibiotic is a man-made antibiotic
• antibiotic production involves a fermentation step
• mutagenesis and gene amplification in microbes are techniques used to boost antibiotic production
• vaccines stimulate the host's immune response to fight a specific pathogen
• microbial biotechnology is used in the production of several kinds of vaccines, such as subunit vaccines and vector-based vaccines

Glossary

Antibiotic - any substance that can destroy or inhibit the growth of bacteria and similar microbes

Antimicrobial agent - a substance that kills or inhibits the growth of microbes such as bacteria, fungi, viruses, and parasites

Bioactive molecule - a substance that interacts with living cells or tissues to influence their behavior, such as promoting growth, differentiation, or healing; examples include growth factors, cytokines, or peptides; also called a bioactive compound

Fermentation - any of many anaerobic biochemical reactions in which an enzyme (or several enzymes produced by a microorganism) catalyses the conversion of one substance into another; especially the conversion (using yeast) of sugars to alcohol or acetic acid with the evolution of carbon dioxide

Gene amplification - results from the insertion of multiple copies of a gene into the genome of an organism

Metabolite - any substance produced by, or taking part in, a metabolic reaction

Mutagen - any physical or chemical agent that causes changes or mutations in the DNA sequence; includes UV radiation and certain chemicals

Mutagenesis - the process by which mutations are induced or occur naturally within an organism's DNA

Plasmid - a small, circular piece of double-stranded DNA found outside of the bacterial chromosome; used in DNA cloning procedures; often used as a vector to introduce foreign DNA into cells

Secondary metabolite - a substance produced by an organisms that are not directly involved in growth, development and reproduction (these basic functions are carried out by primary metabolites)

Vaccine - a biological preparation that stimulates the immune system to protect against a specific disease