## 3 Results and Discussion
### NLR-M2 Results
These peculiarities evidence the possibility of noticeable evolution of even the formalin-fixed culture during one year of storage. Panels (c) and (f) of Figures 3-6 show \(M_{2}\) signals recorded at \(F_{\mathrm{sc}}\) = 0.02 Hz with a relatively small hysteresis and close amplitudes in both samples, \"old\" and \"new\", for all strains. Only the amplitude of the response of the \"old\" sample of the MSR-1 strain somewhat exceeds the average level. Well-defined extrema of the signals located near \(H\) = 0 correspond to a large moment of the magnetic centers and can be attributed to the \"heavy\" fraction. However, as the fitting results show, the response also contains a comparable contribution from the \"light\" fraction with an extremum at \(H\) \(\sim\) 10 kA/m. The increase of \(F_{\mathrm{sc}}\) to 8 Hz is accompanied by an increase of the hysteresis and, in most samples, by a shift in the position of the signal extremum from the region \(H<0\) to the region \(H>0\) in the direct scan curves, as Figures 3a, 4a,d, 5a,d, and 6a show. This indicates the large relaxation time, so that the scan period at 8 Hz is too short for the magnetosome ensemble to reach the thermal equilibrium. A considerable increase of the hysteresis in the response of the \"old\" samples after the one-year storage may be associated with the enlargement of some part of magnetosomes of the \"light\" fraction, due to fusion of their magnetic cores during the destruction of their lipid shells. From Table 1, the magnetic moments of the \"heavy\" fraction vary significantly in different MTB strains. Their values correspond, at most, to the NP size of 25 nm, which is too small for any MTB known [9, 45]. The reason is the field hysteresis in the \(M_{2}\) response, which smears the signal to be processed when averaging between the direct and reverse scans (Equation (8)). As a result, in the vicinity of \(H=0\), the fit curves cannot describe the fast growth of the true signals (Figures 3-5 and 6c,f), and the magnetic moment of the \"heavy\" fraction turned out to be essentially underestimated. This rapid growth of the signals, both in forward and backward scans, can be fairly well described by the response curves simulated for the magnetic moment \(M=1.7\cdot 10^{6}\)\(\mu_{\rm B}\) (hatched peaks in the vicinity of \(H=0\) in Figures 3f, 4c, and 5f) corresponding to the NP size of 40 nm, which is within the range that is typical for magnetosomes [1; 2; 3; 45; 5; 9; 46]. However, magnetosomes form linear chains consisting of dozens of NPs strongly coupled by dipolar forces, and their total magnetic moment is, at least, an order of magnitude larger. The simulation of the nonlinear response from such an array yields a narrow peak centered at \(H=100\) A/m (not shown), well inside the hysteresis region (\(\pm 1\) kA/m). Thus, for the obtained anisotropy energy of the order \(10^{3}\) K, even at the smallest scan frequency used, the low-frequency magnetization dynamics of the array with such a great magnetic moment is still far beyond the SPM regime, and the formalism based on the FP equation may not be applied. To correctly measure this signal, a scan frequency \(F_{\rm sc}\) much lower than 0.02 Hz is needed, yet it is unavailable in the setup used. Its upgrade is intended.