2.1. Analysis of Changes in Antibiotic Susceptibility of S. aureus Strains under the Influence of RMF
The performed analyses confirmed that the bacterial strains used in the experiment belonged to
S. aureus species. Resistance to cefoxitin (determining resistance to methicillin), and the presence of the
mecA gene were confirmed for all analyzed MRSA strains. Accordingly, all strains which were included to the study as MSSA were susceptible to cefoxitin and did not harbor the
mecA gene (
Supplementary Figure S1).
The reference MRSA strain (ATCC 33591) and two clinical isolates (namely MRSA 1 and MRSA 2) were selected for the first phase of the analyses, which aimed to assess the changes in the aforementioned strains’ susceptibility to cefoxitin in the presence of RMF. Because resistance to cefoxitin indicates resistance to most other β-lactams, three other antibiotics from this group were included in this experimental line. The research previously conducted by our research group, as well as the studies of several other authors, revealed that the strength of MF impact (regardless of its type or the phenomenon analyzed in its presence) depends on exposure duration as well as on the intensity and/or the frequency of MF [
19,
20,
32,
43,
47], because these two factors determine the physical characteristics of the magnetic signal [
48,
49].
It should be explained here that in the case of the RMF set-up used in the present study, the frequency of AC determines the MF intensity and, importantly, it is responsible for the physical characteristics of the magnetic wave shape. For this reason, the first analyses were carried out to identify the AC frequency at which the generated RMF induced the greatest changes in antibiotic susceptibility. As the optimal exposure time to the RMF has not been established at this stage of the experiment, seeded bacterial cultures on agar plates with the antibiotic were subjected to RMF for 18 h, because this is the time recommended by EUCAST (2021) [
50] guidelines for antibiogram preparation.
It was shown that the strongest effect in terms of changes in growth inhibition zone diameter was observed at the lowest current frequency enabling the provision of the magnetic characteristic for the RMF (5 Hz) in the applied setup (
Table 1). As shown by the simulative calculations, at 5 Hz the amplitude of the RMF was characterized by a longer period between magnetic induction maximal strength state (100 ms with
Bmax 8.1 mT). In contrast, a significantly lower effect in terms of the changes in antibiotic susceptibility was observed when RMF at 50 Hz (the highest current frequency in applied set up) was used. The RMF at this AC frequency was characterized by a shorter period, with 10 ms time between magnetic induction maximal strength state with
Bmax 8.5 mT (
Figure 1). For the intermediate AC frequencies applied (10 Hz and 25 Hz), the changes in inhibition zones were also observed for most of the antibiotics included in the experiment, although the diameters of the zones were, in most cases, smaller, and in no case greater than those observed at the RMF generated at 5 Hz.
When the MRSA 2 strain was exposed to RMF at 10 Hz in the presence of cefepime, cefuroxime and ceftriaxone, a zone of partial growth around the antibiotic discs was formed. This zone consisted of a bacterial layer, thinner and distinctly demarcated from the staphylococcal lawn grown on the rest of the agar plate (
Supplementary Figure S2). It was confirmed that the discussed layer did not consist of contaminating species but of cells of the staphylococcal strain. A similar result (presence of partial growth zone around the antibiotic disc) was also observed when the same MRSA 2 strain was challenged against cefepime in the presence of RMF at 25 Hz. In turn, when RMF at 50 Hz and three of the aforementioned antibiotics were applied, the staphylococcal lawn formed by MRSA 2 reached the edge of antibiotic discs (no inhibition or reduced growth zones were noticed). It can be thus assumed that the observed effect could be related to RMF activity, because its frequencies (5, 25, 50 Hz) were the only variables in this experiment. However, no such effect of RMF was observed for MRSA 1 and ATCC 33591 strains. At 25 Hz RMF, the zones of their growth inhibition were clean and of similar size as the zones observed at 5 Hz. Such results may be related to the specific features of a given strain exposed to the RMF. It is well-known that sensing stress factors, bacteria switch on new pathways aimed at cell preservation [
51]. In particular, the transposition, which represents an important source of genetic variability, can be induced in bacteria exposed to environmental stress, e.g. to the MF [
49,
52,
53]. By this means, bacteria try to adapt using intra-strain variability. This allows enhancement of the persistence of bacteria and promotion of the selection of clones adapted to the particular stress conditions [
54], e.g., RMF of specific characteristics. The conclusion which can be drawn from the discussed results is that RMF potential to induce changes in the antibiotic susceptibility of MRSA strains is not restricted only to the RMF generated at 5 Hz. Nevertheless, because the most favorable results were obtained at 5 Hz, further studies and analyses were performed in RMF using this frequency.
It is worth noting that in our previous work, a completely different pattern of changes in antimicrobial activity was observed with regard to the applied RMF frequency [
10]. In the above work, we aimed to assess the activity of various antimicrobials (antibiotics and antiseptics) against staphylococcal and pseudomonal biofilms exposed to the RMF generated in the range from 10 to 50 Hz, in which the higher the frequency, the greater the antimicrobial effect observed. We assumed that this effect could be caused by the direct correlation between magnetic induction and mixing effect within the biofilm matrix, which was immersed in a liquid microbiological medium. Our assumption was that the more active the MF (please refer to
Figure 1), the more particles of the antimicrobial could reach and deactivate bacterial cells within biofilm layers. These results are in opposition to those presented in
Table 1. However, the previous study used not only a different biological model (biofilm) but also analyzed other antimicrobials. The above additionally emphasizes the impact of the applied variables on the final outcome of the experiment. The findings of the present study agree, in principle, with the report of Stepanian et al. (2000) [
55] who revealed that the percentage of cell survival was proportional to the increase in EMF frequency. The highest percentage (53%) was observed at 50 Hz and the lowest (20%) at 4 Hz. In other words, the lower the frequency of the EMF, the lower the bacterial survival rate. Therefore, although the results of research of the effects of MFs on microorganisms frequently suggest that the effect is proportional to the intensity or frequency of the generated field, it should not be taken as a binding principle. Contradicting results may be explained, e.g., by the differences in MF wave characteristics at lower and higher frequencies, which act on the bacterial cells as a stress factor and consequently cause disturbances in their development; the exposed cells are alternately (however, with different frequency) subjected to weaker and stronger MF influences. As was already indicated, the amplitude of the RMF at a lower frequency is characterized by a longer period between magnetic induction maximal strength state, whereas at higher frequencies this period is shorter. Simultaneously, the applied AC frequencies (5 Hz
https://youtu.be/YXH5CkArdQ0, accessed on 1 September 2021, 10 Hz
https://youtu.be/4TGUOQOVOLo, accessed on 1 September 2021, 25 Hz
https://youtu.be/8r1AcS0dIP0, accessed on 1 September 2021, 50 Hz
https://youtu.be/gfr7AYqCyh8, accessed on 1 September 2021—please click the links to watch the simulation) generated magnetic flux rotation around the stator with different synchronous speeds of 150 rpm, 300 rpm, 750 rpm and 1500 rpm, respectively (calculations performed on the basis on the manufacturer’s characteristics of the stators).
Previous studies have also indicated the ability of different MF types to disturb microbial structures and to permanently damage cell membranes, presumably by their irreversible electroporation [
45,
46]. Fojt et al. (2004) [
32] explained that a drop in bacterial viability after exposure to MF was caused by an increase in the permeability of ion channels in the cytoplasmic membranes or by the formation of free radicals in bacterial cells. The relationship between the changes in the induction of the MF and the formation of active oxygen and free radicals in bacterial cells was previously demonstrated by Kohno et al. (2000), Fojt et al. (2004) and Jin et al. (2009) [
32,
56,
57]. Therefore, it can be expected that in the case of a fairly homogeneous MF, e.g., the RMF generated at 50 Hz, which is characterized by relatively low MF strength fluctuations, bacteria could adapt more easily to the stressor. Such a mechanism was previously reported by Mittenzwey et al. (1996) [
58] who showed high resistance of different bacteria to MFs due to the intracellular repair systems and self-regulatory mechanisms. The data reported by the above-mentioned researchers, together with the results presented in
Table 1 and
Figure 1, indicate the possibility of an additive interaction between β-lactam antibiotics and RMF. At this stage of the experiment, we assumed that the potential mechanisms behind the observed interaction may include a direct influence of the RMF on antibiotic molecules, bacterial membrane/cell wall or metabolic intercellular processes.
The next stage of the experiment aimed to determine the optimal RMF exposure time to obtain the highest increase in antibiotic susceptibility. Previous papers of our research group [
19,
20] as well as reports of other authors [
44,
59], indicated that the time of magnetic exposure (apart from the intensity, frequency and characteristics of the field wave) is also of key importance with regard to the effect exerted on biological systems. Depending on the exposure time, the MF may have a different effect on bacterial viability, i.e., it may increase it [
40,
41,
42] or reduce it [
32,
34,
35]. In the current study, the cultures with antibiotic discs were exposed to the RMF for a specified time, ranging from 1 to 12 h, and then the plates were transferred to the incubator until the 18 h period of incubation was completed. It was revealed that the zones of bacterial growth inhibition increased with the length of exposure time up to 12 h in most cases (
Table 2). However, in the case of MRSA 1 exposed to ceftriaxone and MRSA 2 exposed to cefuroxime, the increase of growth inhibition stopped after 5 h of magnetic exposure. In turn, the first increase in the diameter of the growth inhibition zones around the antibiotic discs was observed after 2 h of the strain’s exposure to RMF compared to an unexposed strain. This effect was observed in
S. aureus strains ATCC 33591 and MRSA 2 subjected to the activity of cefoxitin, cefepime and ceftriaxone. Nevertheless, in most cases the differences in inhibition zones were visible after 4 h of exposure to the RMF. It should also be explained here that in each of the above cases, the period of exposure to the RMF was shorter than the time needed to observe bacterial growth on the medium. However, in the case of cultures exposed longer, at least by the time after which bacterial growth on the media was already observed (>6 h), it was possible to measure the zones of growth inhibition even before transferring the plates to the incubator. It was found that in cultures exposed for more than 6 h, after further incubation w/o RMF, the inhibition zones were only slightly reduced (<2 mm) compared to the zones measured immediately after exposure to the RMF (
Supplementary Table S1). It was also noticed that in a few cases, after magnetic exposure lasting for less than 6 h, a small number of individual colonies was also observed within the growth inhibition zones (thus they were referred to as zones of partial growth inhibition) (
Supplementary Figure S3). No differences between the zones of growth inhibition measured immediately after RMF exposure compared to the zones remeasured after further incubation w/o RMF was observed only when the exposure lasted at least 10–11 h (depending on the staphylococcal strain), when the cultures were already well-developed (
Table 2). For this reason, the optimal RMF exposure time ensuring the possibility of observing a stable effect was defined for 12 h.
The next part of the experiment focused on changes in the susceptibility of the RMF-exposed bacteria to the various β-lactam antibiotics, including different classes of cephalosporins, carbapenems and penicillins. Although the general mechanism of action of β-lactam antibiotics is similar, there are some differences in their specific activity, e.g., related to different binding sites with the PBP2a protein or the binding energy value [
60]. Harrison et al. (2019) [
61] demonstrated that aminopenicillins (e.g. amoxicillin) bind better to PBP2a than cephalosporins. The size of the particles of individual antibiotics, as well as their charge, may also be important, especially taking into account the possible effects of the MF on charged particles, and thus on the process of their diffusion in the microbiological medium [
62,
63]. Our findings showed that an increase in antibiotic susceptibility in all RMF-exposed cultures was obtained for seven out of the eleven β-lactam antibiotics included in the experiment (
Table 3,
Figure 2,
Supplementary Figure S4). A lack of increase in the diameters of growth inhibition zones in RMF was found only for ceftazidime and amoxicillin (for all three strains analyzed); cefradin (in the case of ATCC 33591 and MRSA 2 strains) and cephalexin (MRSA 2 strain). Unfortunately, we were unable to establish any pattern explaining the relationship between the properties, mechanism of action of the specific types of antibiotics and the observed changes in the zones of growth inhibition under the influence of RMF.
The next part of the study aimed to determine whether the observed effect of increased sensitivity of β-lactam antibiotics caused by RMF exposure depends on the specific concentration of antimicrobial used. Since the disc diffusion method is limited to only one concentration of the antibiotic/disc, for the purposes of this experiment, gradient MIC strips (E-tests) were used. The use of E-tests enabled assessment of the changes in antibiotics susceptibility of bacterial cultures exposed to the RMF (5 Hz) depending on the concentration of the antimicrobials. Eight antibiotics were selected for the study, for which in the disc diffusion test the greatest and the lowest differences in the zones of growth inhibition compared to the unexposed control cultures were obtained. It was found that in each (except one) of the RMF-exposed cultures, a substantial decrease in the MIC value (by at least one order of the antibiotic concentration value marked on the E-test strips) occurred compared to the controls (
Table 4,
Figure 3,
Supplementary Figure S5). The only exception was the RMF-exposed
S. aureus ATCC 33591 cultures with ceftazidime, for which the MIC value was the same as for the control. In turn, the decrease in MIC values of cefoxitin, ceftriaxone, cefuroxime, ceftazidime and cefepime was particularly significant in the case of MRSA 1 and MRSA 2 strains exposed to RMF. The MIC values measured in the control settings were 256 µg/mL, while in the RMF-exposed cultures, MIC values ranged from 6 to 96 µg/mL, depending on the strain and antibiotic. Importantly, the use of more precise E-test methodology allowed to detect the effect displayed by RMF in the case of these antibiotics for which no or minimal changes in growth inhibition zones were observed when the disc diffusion method was applied. These antibiotics were, e.g., amoxicillin (in all RMF exposed cultures) and ceftazidime (in MRSA 1 and MRSA 2 cultures but not in the ATCC 33591 strain). To summarize this part of the results, it should be noted that the lack of the influence of the RMF, observed in the case of several antibiotics in the disk diffusion assay, was related to a low concentration of these antimicrobials in the test discs. Nevertheless, the exposure to RMF did not change the susceptibility level of the ATCC 33591 strain to ceftazidime. This reveals not only the importance of intra-species variability in their answer to the same stimuli but also the necessity of testing a high number of microbial strains to draw proper conclusions from the observed phenomena.
The additional finding from the antibiotic diffusion-based assays was the observation that the zones of growth inhibition obtained in the RMF-exposed cultures retained their characteristic shape of a circle in the disc diffusion method and an ellipse when the MIC strips were used (
Figure 2,
Figure 3, respectively). Importantly, in the case of E-tests, the zones of growth inhibition measured in a straight line from the edge of the strip across to the edge of the bacterial lawn in the RMF-exposed cultures were larger than in the control settings, approximately in the same manner as was previously observed when the disc diffusion test was applied (
Figure 4). This observation is particularly important in the context of the basic methodological assumptions for the applied diffusion-based tests. A demonstration of the disturbances in the diffusion of antibiotics under the influence of RMF at this level of analysis would significantly hinder the correct interpretation of the obtained results. On the other hand, if such a phenomenon occurred, it would provide precious data on a possible interaction of RMF with antibiotics.
To determine whether the observed effect of elevated antibiotic susceptibility in RMF is strain-specific, the research was extended to include another 21 MRSA strains. It is well-established that the MRSA mechanism is mediated by the expression of an alternative of PBP2 protein (called PBP2a) characterized by a low affinity for β-lactam antibiotics, resulting in resistance to most β-lactams. However, PBP2a encoded by the
mecA gene which is carried on a mobile genetic element known as a staphylococcal cassette chromosome
mec (SCC
mec) can be regulated by two independent regulatory systems (
mecI-
mecR-
mecR2 and
blaI-
blaR) and multiple chromosomal genes. As shown by other authors, also PBP1-4 and PBP2a structures are not identical between staphylococcal strains [
60]. Moreover, β-lactam resistance in most MRSA is heterogeneous, meaning that while most cells in a population have low MICs, some fraction can survive at much higher MICs [
64]. Despite the naturally occurring differences between strains and their different susceptibility to cefoxitin (confirmed on the basis of the results obtained in control cultures), our studies showed that all analyzed MRSA strains displayed elevated susceptibility to cefoxitin (
Table 5). In the case of eight of the 20 strains, the diameter of the inhibition zones increased at least by 10 mm, while in the others by at least 4 mm. Of note was the observation that the smaller the zone of growth inhibition under the control conditions, the greater the change in its diameter as a result of RMF exposure.
The next stage of the study aimed to demonstrate whether the changes in susceptibility to β-lactam antibiotics are related to the MRSA mechanism based on the presence of PBP2a protein (whether the effect of changes in susceptibility to β-lactams is observed only in MRSA strains, while in the case of MSSA strains, the changes are not observed) or whether the observed effect is strictly related to the presence of the antibiotic in the microbial culture (changes in antibiotic susceptibility occur in the presence of the antibiotic in the RMF-exposed culture, regardless of the lack of methicillin resistance mechanism). For the purposes of this research, five
S. aureus strains lacking the
mecA gene and showing susceptibility to methicillin in a phenotypic test with the cefoxitin-saturated disc were used (
Supplementary Figure S1). The findings did not show any changes in the diameters of growth inhibition of RMF-exposed (5 Hz) cultures when the disk diffusion test was applied (
Table S2). However, when E-tests were used, for all analyzed MSSA strains differences in MIC values were found between the control and RMF-exposed cultures with cefuroxime and cefepime and, in the case of four strains cultivated with ceftriaxone (in all cases, by one order of concentration value) (
Table 6). In turn, no differences were found for cefoxitin. Therefore, it can be assumed that the observed changes in susceptibility to β-lactam antibiotics observed in the case of MRSA strains under the influence of RMF may, at least to some extent, be related to the presence of the alternative PBP2a protein. On the other hand, it can also be noted that the MIC values in the control cultures were relatively low; therefore, it cannot be excluded that the effect of RMF concerning the changes in antibiotic susceptibility has its limitations in the case of strains showing high susceptibility to the analyzed antibiotics.
2.5. Effect of RMF on Cell Morphology
In the present study we confirmed that the observed RMF effect is related to changes in antibiotic susceptibility but not to the direct bactericidal feature of the RMF (measured by means of quantitative culturing). Moreover, we showed that the presence of RMF does not correlate with changes in the diffusion of antibiotics in the agar medium. The above results encouraged us to perform analyses capable of detection of such subtle phenomena caused by RMF as minor cell damage that could weaken the cell structure without completely destroying it. RMF-exposed cells displayed significant alterations compared to their unexposed counterparts. These changes included mostly the shape and size of cells; also collapse of cell walls and cellular leakage was observed (
Figure 8a,b,
Supplementary Figure S8). Noteworthy is that with the exception of the last type of alteration, of a rather irreversible character, one may assume that changes in such altered cells as those pictured in
Figure 8a (and marked with numbers 2 and 3) could be reversed [
69] if the cells were seeded in a fresh microbiological medium and cultivated without RMF presence. Such a phenomenon largely explains the lack of changes in the number between cultivable bacteria exposed and unexposed to RMF (
Figure 5) and a drop in live cells (dyed with SYTO9) in the RMF-exposed vs unexposed setting (
Figure 6).
The results related to durability of the effect of exposure to RMF (post-exposure effect) showed that bacterial cultures previously exposed to RMF behaved in the same way as the control unexposed cultures, i.e., the antibiotic susceptibility level was the same for both types of cultures (
Supplementary Table S3). Thus, despite the inevitable biases related to the applied methodology, the results of this part of the investigation suggest that RMF acts by partial disintegration of staphylococcal cell walls. This statement is particularly important in the context of issues presented in this article (
Table 1,
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6) because it satisfactorily explains the increased efficacy of β-lactam antibiotics acting against already weakened (as a result of RMF exposure) staphylococcal cell walls. Therefore, another SEM analysis was performed to visualize this hypothetical, boosting effect of the RMF on cefoxitin activity.
Figure 8c shows staphylococcal population (resistant to cefoxitin) grown on agar with the aforementioned antibiotic applied.
Figure 8d shows the same resistant staphylococcal strain grown on agar with cefoxitin introduced, but additionally exposed to the RMF (5 Hz). It can also be seen that the application of the RMF together with cefoxitin translated into explicit morphological changes of staphylococcal cells, including alteration of cell shape, collapse of cell wall, decreased turgor and reduced size (probably effected by cytoplasmic leakage). Noteworthy is that these changes were more ubiquitous in the discussed setting (RMF + antibiotic) than in a setting when only RMF was applied (please compare
Figure 8a with
Figure 8d).
To get additional data concerning RMF impact on staphylococcal cell morphology, cross-sections of bacterial cells (exposed and unexposed to the RMF) were performed using transmission electron microscopy. In
Figure 9 and
Supplementary Figure S9, representative pictures of the clinical strain MRSA 1 are presented. Pictures a,b of
Figure 9 show bacterial cells unexposed to RMF, with an oval shape typical for staphylococci, whereas the RMF-exposed staphylococcal cells present deformed morphology, particularly well-visible in pictures c,d of
Figure 9. The deformation pattern included cell elongation (loss of oval shape, picture c of
Figure 9) or formation of bulges (indicated with red arrows in
Figure 9d,f). As seen in
Figure 9a,b, the walls of unexposed cells are evenly contrasted and strongly distinguished from the cytoplasm, while in RMF-exposed cells a lower contrast was observed between the cytoplasm and the cell wall (marked with green arrows in
Figure 9), indicating a loss of wall density. In the case of RMF-exposed bacteria, contraction of the cytoplasm is also visible, manifested by an uneven border between the cytoplasm and the cell wall (blue arrows in
Figure 9) suggesting lower cytoplasm density. Such a cytoplasmic condensation induced by membrane damage was recently observed in Gram-negative
E. coli by other authors [
70]. The above-mentioned observations suggest that RMF activity translates into a reduction in cell wall density, which in turn manifests itself by cell shape deformations and (at least) partial leakage of cytoplasm.
The following hypothesis is thus worth considering. As already mentioned, MRSA strains carrying the
mecA gene encoding an alternative form of PBP, called PBP2a, have a reduced (to a varying extent) affinity for β-lactam antibiotics [
61,
71]. Therefore, PBP2 and PBP2a are not the same in different strains (the polymorphism of the
mecA complex may affect the function of these genes and methicillin resistance mechanism) and thus these proteins can show different affinity for the same β-lactam antibiotic. The EUCAST defines the presence of methicillin resistance when the inhibition zone is ≥22 mm, not only when there is no inhibition zone. This means that binding of β-lactam antibiotics to PBP2a protein occurs and translates to a certain level of inhibition of peptidoglycan synthesis in MRSA strains. The process of binding, and the aforementioned inhibition, are significantly less effective compared to MSSA strains expressing unmodified PBP protein, as well as compared to the competitive reaction with peptidoglycan chains. Thus, given the competitive mechanism, a large proportion of the PBP2a is not blocked and is involved in the cross-linking of the peptidoglycan chains, and so the cell survives (at least in relatively low concentrations of the β-lactam antibiotic). Considering the observations using SEM, TEM and fluorescence microscopy suggesting that the RMF induced disturbances in the structure of the bacterial cell wall, it can be assumed that the small amount of β-lactam antibiotic, which blocks the activity of some of the entire PBP2/PBP2a protein present in the MRSA cell, is sufficient to induce further structural changes in the cell wall, which, in effect, makes it impossible to maintain the intracellular osmotic pressure and leads to cell disintegration. Additionally, considering one of the RMF interaction mechanisms associated with increasing the mixing efficiency [
63], which apply to both the external and internal environment of the cell culture and cells, it can also be assumed that in the RMF-exposed bacteria there is an additional increase in the pressure of the external and intracellular liquid on the cell wall that may lead to its further damage.