By Elizabeth Ohneck, PhD
The introduction of penicillin to the clinic in the 1940s ushered in an era of hope in our battle against bacterial disease. Penicillin, produced by a lowly mold, Penicillium notatum, was accidentally stumbled upon by Dr. Alexander Fleming in 1928, when he noticed areas of inhibited bacterial growth around the mold, which had contaminated a petri dish of Staphylococcus aureus. A team of researchers led by Dr. Howard Florey later purified the penicillin compound and developed a method for large-scale production. By 1944, penicillin was being mass-produced and commercial production methods refined. It was a powerful tool during WWII, drastically reducing deaths from infected wounds and surgical procedures. Finally made broadly available to the public in 1945, penicillin was heralded as a “magic bullet,” capable of curing a wide variety of bacterial diseases. For an excellent history of the discovery and development of penicillin, I recommend the book The Mold in Dr. Florey’s Coat: The Story of the Penicillin Miracle, by Eric Lax.
Just 4 years later, strains of bacteria resistant to penicillin began to appear, spurring a search for alternatives. Following the penicillin model, researchers turned to other microorganisms for potential antibiotic substances. Soil bacteria provided a rich reservoir for such compounds, leading to the discovery of antibiotics such as streptomycin, erythromycin, and cephalosporins. But carelessness, complacency, and overuse of antibiotics allowed the pathogens to fight back, developing ways to resist these new drugs and landing us in our current predicament, facing dangerous strains of bacteria resistant to almost all available antibiotics, such as MRSA (methicillin-resistant S. aureus), MDR-TB (multi-drug-resistant Mycobacterium tuberculosis), and Neisseria gonorrhoeae. Only a small percentage of soil bacteria – approximately 1% – are able to grow under normal laboratory conditions, and with this reservoir seemingly exhausted, biochemical laboratory synthesis efforts failing, and no new antibiotics in the pipeline, the need for new antibiotic candidates is obvious.
One team of researchers decided to go back to the soil. In an exciting paper published in Nature this month, Ling et al. describe the development of a novel method for growing previously uncultivable soil bacteria, leading to the discovery of a new antibiotic they have called teixobactin. To grow the finicky soil bacteria, a sample of soil was diluted and distributed into a multichannel device called an iChip so that each channel contained only one bacterium. The channels were covered with semipermeable membranes to keep the bacteria in but allow exchange of nutrients and growth factors between the channel and the environment. The iChip was then returned to the original soil from which the sample was taken, creating growth conditions highly similar to the bacteria’s native environment. This method increased the number of soil bacteria that could be cultured from 1% to nearly 50%. Even better, once a colony is established in the iChip, many of the bacteria can then be grown in a laboratory setting.
The researchers screened extracts from 10,000 isolates obtained by this method for antimicrobial activity against S. aureus and identified a new protein compound produced by the Gram-negative organism Eleftheria terrae, which they named teixobactin. Teixobactin showed potent activity against M. tuberculosis and Gram-positive pathogens such as Clostridium difficile (which causes intestinal infection sometimes referred to as “C. dif”), Bacillus antrhacis (which produces the causative agent of anthrax), and S. aureus, including drug-resistant strains. Unfortunately, it was not effective against Gram-negative bacteria. Teixobactin showed no toxicity against mammalian cells in culture, and effectively cleared S. aureus and Streptococcus pneumoniae infections in mice.
Through several biochemical investigations, the researchers determined that teixobactin inhibits production of the bacterial cell wall by binding to highly conserved cell wall building blocks. Without a cell wall, bacterial cells lyse, spilling their essential contents into the environment. Other antibiotics, including penicillin, that inhibit the cell wall target its protein components. One mechanism of antibiotic resistance, known as target modification, results when these protein components, encoded in the bacterial DNA, acquire changes from random DNA mutations that alter the antibiotic target site, preventing antibiotic recognition and action. But teixobactin recognizes a lipid (fat) molecule, suggesting it could be less likely that resistance will develop in this manner. Excitingly, growth of S. aureus and M. tuberculosis on low doses of the drug and passage of S. aureus in the presence of subinhibitory concentrations for 27 days did not result in resistant isolates. In contrast, resistance to the antibiotic ofloxacin began emerging after only 3 days of serial passaging. (Research note: Oflaxacin targets DNA gyrase, an important enzyme in DNA replication and transcription. It would have been nice to see the serial passaging experiment done with penicillin and vancomycin, the antibiotic the researchers compare teixobactin to throughout the paper, as this would have provided control data related to other cell wall targeting antibiotics.) The potent effectiveness, lack of observed toxicity, and failure of immediate resistance development make teixobactin a good candidate for a new antibiotic.
These findings have been well celebrated in the media, with the suggestion that teixobactin might be a new “magic bullet”, its “resistance to resistance” finally giving us the upper hand in the battle against antibiotic resistance. It is important to keep in mind, however, that this drug is not effective against Gram-negative bacteria, which have a second membrane that protects the cell wall components which teixobactin targets. Thus, multi-drug resistant strains of pathogens such as E. coli, Pseudomonas aeruginosa, and N. gonorrhoeae are left to wreak havoc in the clinic. More importantly, claims of teixobactin’s “resistance to resistance” are a bit premature. Target modification is only one mechanism by which bacteria can gain antibiotic resistance. Other mechanisms include the acquisition of an enzyme that destroys or modifies the antibiotic, alteration of cell metabolism to avoid use of the product or pathway that the antibiotic targets, changes in cell wall composition or structure that keep the drug out of the cell, or use of an efflux pump, a cellular machine that can recognize and pump out antimicrobial compounds. While some of these mechanisms are more complex and may require exchange of DNA with other bacteria to acquire, all are possible. Development of resistance by serial passaging of a single strain in the lab is much different than within a human patient, where interactions with other pathogens and the normal flora (the bacteria normally found on and in the human body) are plentiful, and the presence of other antimicrobial compounds and alternative nutrient sources create a complex environment that could have significant effects on the development of resistance. There is evidence that genes for antibiotic resistance can be transferred among bacterial species, including those of our normal flora, in vivo.
The authors do acknowledge that resistance may develop, but assert that it would likely take several decades, citing the 30 years it took for development of resistance to vancomycin, which works by a mechanism similar to teixobactin, as an example. The comparison of the development of vancomycin resistance to that of other antibiotics, however, isn’t entirely fair. When vancomycin was first introduced, its relatively high toxicity and low efficacy kept it as an antibiotic reserved only for patients with allergies to β-lactams (such as penicillin) or resistant infections. In the early 1980s, as resistance to other antibiotics became more prevalent, vancomycin use spiked, and was followed by the development of resistant strains by 1986. Thus, the case of vancomycin resistance should actually serve as a warning: should teixobactin prove a viable antibiotic, careless overuse will quickly relegate it to the ever-growing pile of ineffective antibiotics.
Still, the significance of these findings should not be overlooked or understated. Ling et al. have provided us with the first truly promising antibiotic candidate in many years. Perhaps more importantly, they have developed a method for growing bacteria previously uncultivable in the lab, not only expanding the pool of available antibiotic candidates, but also creating a tool that could prove revolutionary in microbiology, ecology, and environmental research. We should be hopeful that teixobactin is safe and effective in human trials, and that either it can be modified to be effective against Gram-negative pathogens or that the iChip method reveals another compound as potent as teixobactin for their treatment. But we must also be responsible and cautious, so as not to squander these precious new drugs and hasten an era of untreatable bacterial diseases.