Figure 1 indicates that the frequency of co-detection between the 5 respiratory pathogens and TRGs continues to rise over time. One trend observed in Figure 1 is the periodicity, where the co-detection rates of TRGs and each pathogen reach a maximum during the middle of each year. The frequency of co-detection is the highest in the summer and the lowest in the winter of each year. Various literature report a seasonal incidence of antibiotic resistance in various pathogens [31–33]. It is possible that this trend occurs from fluctuation in the number of tests physicians administer to patients during different times of the year. Alternatively, the seasonal trend here may be occurring because of the different levels of antibiotics prescribed to patients throughout the each season, especially during the flu season . The CDC considers the respiratory illness/flu season in the United States to begin in October and end in May, peaking in February . As a result of the rise in number of antibiotics prescribed during February, an increased number of pathogens activate the production of TRGs . Throughout spring months, TRG up-regulation and transmission continues to increase until months after flu season subsides. As a result, the rise in TRG co-detection frequency seen each summer could be a delayed effect from the increase in tetracycline exposure from the rise in number of antibiotic prescriptions that are administered during early winter/spring of each year. Moreover, these seasonal data could also have a clinical impact as drug therapy could be periodically altered and antibiotic effectiveness could be increased. Physicians may have greater success prescribing tetracycline in the winter when co-detection frequency is lower, than in the summer when co-detection is at maximum levels. Seasonal trends could be occurring with other first-line antibiotics as well, but more studies are needed to test this hypothesis.
Figure 2 illustrates that all 4 age groups rose in co-detection frequency over time, but the frequencies of co-detection follow different trends for the age groups. Across the 5 pathogens, patients age (0–2] display the lowest frequency of co-detection of TRGs and respiratory pathogens, while patients age (13–50] demonstrate the highest rate of co-detection. These data suggest that tetracycline usage can be optimized in different age groups over the yearly cycle. Even though tetracycline has the highest chance of being effective in patients age (0–2], physicians typically avoid prescribing tetracycline to younger children, as the treatment can cause staining of permanent teeth [36, 37]. Once dental maturity is reached, the staining of teeth from tetracycline use is no longer an issue. The avoidance of prescribing tetracycline to infant patients could be an underlying reason for the lower co-detection measurement in the youngest age bracket. Alternatively, one approved use of tetracycline is to combat dermatological pathogens and is a therapeutic option for treating young adults with acne [31, 34]. The increased utilization of tetracycline in teenage patients could be an underlying cause for the higher TRG co-detection frequency observed in the (13–50] age group. In addition, environmental factors such as diet may have an effect on the large co-detection frequency of both (13–50] and (50–100] age groups. Since over half the antibiotics produced for use in the United States are utilized for agricultural purposes, it may be plausible to hypothesize that the longer a person ingests tetracycline-infused foods, the greater likelihood that normal microbial flora build an immunity to the antibiotic. Then, when a respiratory pathogen infects a patient, these flora can pass tetracycline genes to the foreign pathogen via conjugation .
There are two possible explanations for why a longitudinal effect is observed in Figure 3. Physicians in the Northeast could be prescribing tetracycline to patients at higher rates than in other areas of the country. Alternatively, various diets and varieties of food eaten across the United States could have an impact on different frequencies of TRG and respiratory pathogen co-detection. An underlying reason for the varying frequencies of TRG co-detection throughout different regions of the United States could be because of geographical differences regarding tetracycline application in agriculture. The only outlier in the Western states is Nevada, which has one of the largest rates of co-detection, but is surrounded by states that have the lowest levels of co-detection of TRGs and respiratory pathogens. There is no apparent hypothesis for why Nevada has a large co-detection frequency. Taken together, these data suggest an environmental influence, but a subsequent analysis of data estimating per-state tetracycline loads would be needed to correlate these data to co-detection frequencies.
There are two common causes for increased tetracycline exposure. The first includes excess prescriptions being provided to patients by physicians and the other involves overexposure in agriculture and livestock feed. It is likely that these pathogens contained genes and pathways that predated the antibiotic era that could neutralize tetracycline [39, 40]. Increased utilization in the fields of agriculture and medicine allowed these time-weighted genetic determinants such as synthesis of inactivation enzymes, efflux pumps, or ribosomal protection proteins to be up-regulated and passed to other pathogens via plasmid transfer [39–42]. S. aureus, MRSA, S. pneumonia, H. influenzae, and M. catarrhalis are all found in proximity in the upper respiratory tract of most humans, therefore, it is plausible that these pathogens exchange genetic information frequently. The genomes of many pathogens contain TRGs with the three major genes including tet (K), tet (M), and tet (O) [1, 15, 16]. In this study, only genetic targets for tet (K) and tet (M) were included in testing. With the exclusion of tet (O) and other tet genes, it can be speculated that the actual rate of tetracycline resistance in patients is even higher than seen here.
A strong hypothesis for the high frequencies of TRG co-detection is that the high tetracycline use in agriculture facilitates tetracycline resistant gene activations, which is then passed to other pathogens via conjugation. The rampant use of antibiotics is a more pressing issue in the United States than in other countries because over half the antibiotics that are produced in the United States are used for agricultural purposes, an extremely high percentage [3, 4]. For many years it has been disputed whether or not antibiotic use in agriculture is responsible for the rise in the number of antibiotic resistant pathogens. The FDA argues antibiotic levels recommended for agricultural use have been tested in a laboratory setting on bacteria in culture, with very low frequencies of pathogens acquiring resistance . However, some literature demonstrates that sub-inhibitory antibiotic exposure can cause development of resistance over time [44–47]. Recently, the FDA has begun to regulate the amount of antibiotics utilized in the agricultural and medical industries . Playing close attention to the levels of pathogens that are becoming resistant to antibiotics is imperative because throughout the past 15 years only 9 new antibiotics have been approved to combat certain illnesses . Since there are a limited number of new antibiotic medications in the developmental pipeline, it is important to monitor levels of tetracycline resistant pathogens so that the medication can continue to be an effective treatment for acne and uncomplicated community-acquired MRSA [48–50].