The mounting evidence of resistance to drugs
A large body of evidence had already been collected on resistance to drugs in both the laboratory and clinic long before penicillin became a drug and Fleming and Dubos issued their warnings. One of the earliest to observe the phenomenon was Paul Ehrlich, a German chemist who helped to synthesise Salvarsan (arsphenamine), the first compound found to have antimicrobial activity. In 1907 he noticed that repeated doses with Salvarsan activated resistance when he tested out its action in mice infected with Trypanosoma brucei, the parasite that causes sleeping sickness. Ehrlich hypothesised that a regime of alternating antimicrobial agents might be one way to overcome such resistance (Kazanjian; Moberg).
One of the other antimicrobial therapies then emerging was ethylhydrocupreine (optochin), a derivative of quinine which was then the most potent antimicrobial substance known against pneumococci. Nevertheless, it did not take long before AMR was also spotted in conjunction with this drug. In 1910, Julius Morgenroth and Richard Levy, two German scientists, found that mice infected with the pneumococcal bacteria became unresponsive to optochin. The phenomenon was not only observed in mice. Four years after optochin was licensed for treating pneumonia in humans (1913), physicians found the same problem developed in patients. Subsequent experiments revealed resistance could develop within only one or two days' exposure to the drug (Ash, Solis-Cohen; Kazanjian).
Figure 3.1: Two dimensional structure of Optochin (Credit: Wikipedia) Optochin (ethylhydrocupreine hydrochloride) is a synthetic derivative of quinine. It was first shown by Julius Morgenroth and Richard Levy to protect mice against pneumococcal infection in 1911.
The difficulties of resistance observed with optochin was just the start of what would become a familiar pattern for subsequent antimicrobial agents. This included Salvarsan, the first antimicrobial introduced on to the market in 1910. Proving highly effective in the treatment of syphilis, a bacterial infection primarily spread through sexual contact, Salvarsan rapidly became the most widely prescribed drug in the world. Before Salvarsan, syphilis was treated with toxic mercury compounds which caused side-effects that were often much worse than the actual disease. The first instance of resistance to Salvarsan was recorded in a patient with syphilis in February 1924 (Stekel). Click here for more on Salvarsan.
Resistance also developed very fast to the next generation of antimicrobial drugs - sulphonamides, also known as sulpha drugs. The first case of resistance to sulphonamides was reported in a male patient with gonorrhoea in 1938. This was just two years after the first sulpha drug, Prontosil, became available on prescription. The problem was not confined to gonorrhoea. In the winter of 1944-45 streptococci infections resistant to sulphonamides reached epidemic proportions after American sailors from Great Lake Naval Centre in Illinois travelled across the world with sore throats unresponsive to sulphonamides. The cause of the AMR was put down to the mass distribution of such drugs by the US military to prevent disease (Moberg; Leach). Click here for more on sulphonamides.
The widespread emergence of resistance to sulphonamides was in fact a key driver behind the development of penicillin. Yet, there were early signs that the same problem would also plague penicillin. Well before penicillin began to be explored in earnest for clinical treatment, Fleming noted in his seminal paper of 1929 that penicillin could not inhibit the growth of Balantidium coli (B. coli), a type of Gram-negative bacteria now called Escherichia coli, and a number of other bacteria belonging to the Salmonella species (Fleming, 1929). The issue was again flagged up in 1940 by Ernst Chain and Edward Abraham, two biochemists behind the development of penicillin in Oxford for clinical use. In particular, they found that B. coli and other microbes, such as Staphylococcus aureus (S. aureus), were able to produce an enzyme that could inactivate penicillin. They called the enzyme 'penicillinase' (Abraham, Chain).
Resistance also manifested itself when the Oxford team injected purified penicillin for the first time into mice and the first patient, Albert Alexander. Both the mice and Alexander died as soon as treatment stopped. In Alexander's case he could not have the full course of treatment because the penicillin supply dried up. He died of staphylococcal septicaemia. The manifestation of resistance in both the mice and Alexander revealed that staphylococci infections could only be cured if sufficient penicillin was given (Chain, Florey, Adelaide, et al).
Figure 3.2: Albert Alexander, the first patient to receive purified penicillin (Credit: Linda LeBlanc). Alexander worked as a policeman in Oxford. Most stories told about Alexander indicate that he first fell ill after getting a scratch on his face from a rose bush in his garden which got infected with streptococci and staphylococci. The infection then spread to his eyes and scalp. In 2010, however, the discovery of some old police pamphlets suggested that Alexander's infection might instead have been caused by some injuries he suffered from a bombing raid when he was seconded from Abingdon to Southampton. He was shifted to the Radcliffe Infirmary in Oxford when his infection became severe. Florey and Chain decided to treat Alexander after learning about his case at their Oxford College high table one evening (Wood; Markel).
Figure 3.3: Extract from Abraham, Chain, Fletcher. This extract outlines the course of penicillin treatment given to Albert Alexander, the first patient treated by the Oxford team with purified penicillin. It records that Alexander's infection previously failed to respond to a sulpha drug, but that his condition improved 24 hours after receiving penicillin. Alexander received penicillin for 5 days until the supply of the drug dried up. Encouragingly Alexander's condition stabilised for 10 days, but he then went downhill because the infection returned. He died a month later.
The phenomenon of resistance was not only observed by the Oxford team. In December 1942 Charles Rammelkamp and Thelma Maxon, two researchers at Boston University, demonstrated through laboratory experiments that S. aureus bacteria could develop resistance to penicillin by exposing successive generations to weak dilutions of the drug and it correspondingly took stronger and stronger doses to destroy them. Rammelkamp and Maxon also found a similar pattern among patients who were given the drug in a trial, where four out of fourteen patients were found not to respond to the standard dose of penicillin. In one patient, a 47 year old man with a deep bone infection, this resistance increased sixty-four times over the course of his eighteen days of treatment (Rammelkamp, Maxon; McKenna).
Many more articles appeared during the early 1940s reporting penicillin resistance in S. aureus bacteria (Beigelman, Rantz). One investigation, published in June 1944 by William Kirby, a physician at Stanford University, found penicillin-resistant S. aureus bacteria was not confined to patients who received the antibiotic (Kirby). By 1950 a number of other researchers had also demonstrated through laboratory experiments the development of penicillin resistance in a variety of bacteria, including pneumococci, streptococci, meningococci, gonococci and Actinomyces bovis. Overall staphylococci bacteria tended to develop the fastest resistance (Miller, Bohnhoff).
Figure 3.4: Electron micrograph of Staphylococcus aureus bacteria stained purple, at the high magnification of 20,000 (Credit: Janice Haney Carr, Centers for Disease Control and Prevention's Public Health Image Library). S. aureus is a Gram-positive, sphere-shaped (coccal) type of bacteria. Often referred to as a 'staph infection' or 'staph bacteria, S. aureus was first discovered by Alexander Ogston in 1880 when he began investigating high mortality rates in post surgery patients. About 30 per cent of the population carry S. aureus in their noses or on their skin without knowing about it. The presence of bacteria on the skin can be beneficial to humans because it can potentially aid the expansion of the memory of T-cells. However, it can also be extremely dangerous if it infects the systemic components of the body. S. aureus can cause a multitude of diseases, some of which have fatal consequences. Among the problems that staph bacteria can cause are: skin infections, sepsis, mastitis and respiratory infections like pneumonia. Initially staph infections were fought with penicillin. Following the emergence of penicillin resistance in staph bacteria, the antibiotic methicillin was introduced. The drug was used from 1961. It did not take long, however, for the bacteria to develop resistance to methicillin, a pattern that was soon repeated with a number of other penicillin-related antibiotics. Called methicillin-resistant S. aureus, or MRSA, such bacteria, also known as superbugs, first emerged in the early 1960s. MRSA infections are now a worldwide problem in clinical medicine.
Despite the early warnings, physicians rapidly embraced penicillin on a large-scale. They were encouraged to do so in part because it carried less toxicity than previous antimicrobial agents. This meant that it could be used to treat internal as well as external infections. But resistance swiftly emerged to the drug.
Just how fast the problem developed can be seen from the case of the work of Mary Barber, an English bacteriologist, who tracked an increasing frequency of penicillin-resistant staphylococci strains in specimens taken from patients treated in Hammersmith Hospital in the 1940s. Notably, she found very few penicillin-resistant strains before 1944. This, however, soon changed. The incidence of penicillin-resistant strains rose from 14.1 to 38 per cent between April 1946 and June 1947. By 1948 Barber reported that the incidence had risen to 59 per cent. She and colleagues found resistant strains in samples from patients with a variety of infections, including septicemia, superficial skin lesions, conjunctivitis, aural, and nasopharyngeal infections (Barber, 1947; Barber, 1948; Barber Rozwadowska-Dowzenko). Click here for more information about Barber.
While penicillin-resistant staphylococcus strains were found to be widespread in hospitals by the early 1950s, for many years it was assumed that such resistance was low in the community. This belief was first challenged by a Danish study published in 1969, which found such resistance to be just as common in the community as it was in the hospital. The same phenomenon was soon also uncovered elsewhere. One population study, conducted in the United States in 1972, revealed that 68 per cent of S. aureus strains carried by 47 per cent of healthy school-aged children under 10 years of age were penicillin resistant (Chambers).
Figure 3.5: Trends of S. aureus resistance to penicillin and methicillin (Credit: McDonald, figure 1). Key: Gray Squares = community strains. Black Squares = hospital strains.
The graph shows that the percentage of S. aureus strains that were resistant to penicillin and methicillin developed fastest in the hospital environment and then moved into the community. However, the trend had merged by the early twenty-first century.
As was the case with Salvarsan and sulphonamides at the beginning of the twentieth century, many scientists and physicians initially thought that it would be possible to counter the rise of bacterial resistance to penicillin by creating new antimicrobial agents. Their optimism was buoyed by the release of many new antibiotics onto the market. Thirty-seven such new drugs were marketed between 1945 and 1975. (Click here to go to Figure 1.3 in the introduction).
The appearance of new antibiotics, however, did not solve the problem. As figure 3.6 shows, resistance emerged with each generation of new antibiotics. Yet, despite this recurrence physicians and scientists did not grasp the full significance of AMR until the early 1970s. By then it was clear that a number of strains of S. aureus, Streptococcus pneumoniae, Pseudomonas aeruginosa, and Mycobacterium tuberculosis had become resistant to multiple antibiotics (Lowy). Staphylococci bacteria were especially quick to develop resistance. Figure 3.7 indicates such resistance took three years to develop in the United States in the case of penicillin and methicillin, and then just one year in the case of linezolid and ceftaroline. Multi-drug resistance in S. aureus is now the norm. This is particularly worrying because of the diverse number of life-threatening diseases with which it is associated.
Figure 3.6: Timeline for when key classes of antibiotics were first discovered and resistance to them was first reported.
Figure 3.7: First reported cases of bacterial resistance against key antibiotics in the U.S. (Credit: Center for Disease Dynamics & Economics Policy, US CDC 'Antibiotics resistance in US', 2013).
Staphylococci are not the only resistant pathogens that are of concern today. Figure 3.8 provides a list of some of the bacteria singled out by WHO and the Longitude Prize (a British charity) as in urgent need of new drugs to combat AMR. The list illustrates how far AMR has spread among different bacteria. WHO divided the pathogens into three priority categories: critical, high and medium. Its critical list includes multidrug resistant bacteria that pose a significant threat in hospitals, nursing homes and patients who are dependent on devices such as ventilators and catheters. Those in WHO's high-medium category are bacteria which are showing increasing resistance against the main drugs used to treat them. Some of the bacteria listed, such as Mycobacterium tuberculosis, already pose a significant challenge in their own right and will only become harder to treat as their resistance to drugs increases.
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