Jung Tsai
Bacteria are among the simplest yet most powerful forms of life. Most are single-celled, ancient, and remarkably resilient. Long before complex organisms appeared, bacteria existed as the original life forms on Earth. They are everywhere—on land, in the oceans, in the air, and within us. In fact, each human body contains about 40 trillion human cells, yet the number of bacteria living inside us is double or even triple that amount.
For a long time, scientists asked a simple but profound question: Why do we harbor so many bacteria? The answer has become increasingly clear—bacteria are not merely passengers; they are essential partners in our survival. They play a crucial role in digestion, immunity, metabolism, and even mental health. This realization has fueled the rapid growth of probiotics (beneficial bacteria) and prebiotics (fibers that nourish them), now a massive global industry. Properly balanced, bacteria assist almost every function of the human body.
Yet bacteria also have a darker history. In the early 20th century, bacterial infections killed nearly half of the world’s population. Diseases such as cholera, smallpox, typhoid fever, syphilis, and the Spanish flu devastated humanity. Without medical intervention, life could come to a standstill. Understandably, scientists and physicians once believed that if we could eliminate bacteria, we would conquer infectious disease forever.
A turning point came in 1928, when Alexander Fleming of England observed that a fungus contaminating a Petri dish had killed surrounding bacteria. That fungus produced penicillin. Unfortunately, due to medical inaction and limited technology, penicillin was not widely used to treat pneumonia until a decade later. From that moment on, generations of antibiotics—including cephalosporins—were developed, dramatically reducing deaths from infection. Infection was no longer the leading cause of mortality.
However, our relationship with bacteria is far more complex than simple warfare.
Early last century, a Ukrainian scientist—later awarded the Nobel Prize in 1908—discovered that Lactobacillus fermentation produced yogurt associated with exceptional longevity among Bulgarian villagers. He was a century ahead of our time, recognizing that certain bacteria could promote health rather than destroy it.
At birth, a baby’s body is sterile. Beneficial bacteria are acquired through vaginal delivery, skin contact, crawling, and breastfeeding. This is why cesarean sections and bottle feeding, while sometimes necessary, may deprive infants of early microbial exposure. Japanese studies have shown that bacteria-free mice are less healthy and die prematurely, a finding confirmed across many animal models. Simply put, life without bacteria is not sustainable.
Bacteria are also indispensable in modern medicine. Through genetic recombination, they enable the large-scale production of insulin, saving millions of diabetic patients worldwide.
From an evolutionary perspective, bacteria possess a remarkable advantage. While genetic mutations through traditional, linear evolution may take hundreds of millions of years, bacteria can exchange genes horizontally within days. This ability allows them to rapidly develop resistance, creating so-called “superbugs” that render antibiotics ineffective. Ironically, scientists now seek to mimic this bacterial gene-jumping process for medical innovation.
During my surgical career, two powerful examples profoundly reshaped our understanding of bacteria:
First, gastric and duodenal ulcers.
For over half a century, ulcers were blamed on stress and excessive stomach acid. Patients underwent acid suppression, vagotomy, or even gastrectomy. Then two Australian pathologists, Drs. Barry Marshall and Robin Warren, identified Helicobacter pylori in gastric specimens. To prove their theory, they famously swallowed the bacteria themselves and developed ulcers within days. Their discovery revolutionized treatment and earned them the Nobel Prize in 2005. Today, over 90% of ulcers are known to be caused by H. pylori and are treated with antibiotics and acid suppression—surgery is no longer required.
Second, toxic megacolon.
This devastating condition is caused by overgrowth of Clostridium difficile, often following antibiotic abuse, particularly in elderly patients. The resulting imbalance of gut bacteria leads to massive gas production, colon rupture, and mortality rates exceeding 50%—even after emergency total colectomy. Once again, the enemy was not bacteria alone, but the destruction of bacterial balance.
Today, innovative treatments such as fecal microbiota transplantation—using stool from healthy young donors—have shown remarkable success in restoring gut health in elderly patients. Institutions like the Cleveland Clinic have become world leaders in this field. These therapies were inspired by experiments in which old and young mice shared the same environment, leading to rejuvenation of the older animals.
In conclusion, bacteria are neither purely foes nor merely friends—they are essential companions. There is still much to learn from them, and we have no choice but to coexist. The future of medicine lies not in eradicating bacteria, but in understanding, respecting, and partnering with them.
<2026-01-18>