Improved Methods for Mass Production of Magnetosomes and Applications: a Review Abdul Basit1,2, Jiaojiao Wang1, Fangfang Guo3, Wei Niu1 and Wei Jiang1*

Basit et al. Microb Cell Fact (2020) 19:197 https://doi.org/10.1186/s12934-020-01455-5 Microbial Cell Factories

REVIEW Open Access Improved methods for mass production of magnetosomes and applications: a review Abdul Basit1,2, Jiaojiao Wang1, Fangfang Guo3, Wei Niu1 and Wei Jiang1*

Abstract Magnetotactic bacteria have the unique ability to synthesize magnetosomes (nano-sized magnetite or greigite crystals arranged in chain-like structures) in a variety of shapes and sizes. The chain alignment of magnetosomes enables magnetotactic bacteria to sense and orient themselves along geomagnetic felds. There is steadily increasing demand for magnetosomes in the areas of biotechnology, biomedicine, and environmental protection. Practical difculties in cultivating magnetotactic bacteria and achieving consistent, high-yield magnetosome production under artifcial environmental conditions have presented an obstacle to successful development of magnetosome applications in commercial areas. Here, we review information on magnetosome biosynthesis and strategies for enhancement of bacterial cell growth and magnetosome formation, and implications for improvement of magnetosome yield on a laboratory scale and mass-production (commercial or industrial) scale. Keywords: Magnetosome biosynthesis, high-yield, cell growth, culture conditions, commercial applications

Introduction structures, are synthesized by a variety of magnetotactic Te research interest of nanoparticles applications in the bacteria [4]. Many recent studies have involved cultured felds of biomedical, biotechnology and environmental magnetotactic bacterial strains, particularly Magneto- protection has gained tremendous importance in recent spirillum gryphiswaldense MSR-1, Magnetospirillum years. Te development of magnetic nanoparticles is the magneticum AMB-1, Magnetospirillum magnetotacticum outcome of that investigative focus and signifcance [1]. MS-1 and Magnetospirillum sp. ME-1 [5, 6]. Investiga- Te physico-chemical procedures for the preparation of tions of these strains have greatly increased our under- magnetic nanoparticles involve high cost and chemicals standing of molecular mechanisms of magnetotactic with environmental implications and human health haz- bacteria in general [5]. Chemical composition, morphol- ards. Tus, the need arises to ascertain and use environ- ogy, and size of magnetite crystals are uniform within a mental friendly, biocompatible, cheap, and low energy given strain, but difer among strains of magnetotactic demanding methods for preparation of nano-particles bacteria. Magnetite and greigite crystals of magneto- [2]. In this situation, the nano-particles synthesized by somes generally display constant shape, although slight magnetotactic bacteria with distinctive characteristics variations of size and shape are sometimes observed for would be right choice and requisite for biomedical and greigite [7]. biotechnology applications [3]. Magnetosomes, which Considerable research efort has therefore been focused consist of membrane-enveloped, nano-sized magnetite on production of high-quantity magnetosomes [8, 9]. (Fe 3O4) or greigite (Fe 3S4) crystals arranged in chain-like However, their cultivation under experimental conditions has been difcult because of their highly precise *Correspondence: [email protected] and restricted living conditions [10]. Isolation, identifca- 1 State Key Laboratory of Agro‑Biotechnology, College of Biological tion, and characterization of magnetosomes are generally Sciences, China Agricultural University, Beijing 100193, China at an early research stage. Te major obstacle to mass- Full list of author information is available at the end of the article production (commercial or industrial) scale cultivation/

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growth of magnetotactic bacteria (MSR-1, AMB-1, MS-1 phase), the magnetosomes present diferences in size and and ME-1) is the need for high magnetosome yield at stoichiometry [8]. Optimal culture conditions (in regard reasonable expense and energy cost [11, 12]. In theory, to dissolved oxygen concentration, pH, nutrient com- magnetosome yield can be improved through modif- position, and salt concentrations) are highly restrictive. cation (optimization) of culture medium composition A variety of control strategies have been evaluated for and growth conditions [12, 13]. However, few studies achieving more efcient magnetosome yield. Optimized have focused on such optimization. Efects on growth culture conditions for growth of MSR-1, based on studies of changes in factors such as temperature, pH, dissolved to date, are summarized in Fig. 1. oxygen concentration, and concentrations of various salts and acids have repeatedly been investigated, but greater Factors afecting the mass production emphasis is needed on maximization of magnetosome of magnetosomes yield [12]. Here, we review information on magneto- Nutrient‑balanced feeding some biosynthesis and strategies used for enhancement Te major factor afecting growth of magnetotactic bac- of bacterial cell growth and magnetosome formation, and teria, and consequently magnetosome formation, is con- implications for improvement of magnetosome yield on centration of nutrients, particularly carbon source. It was a laboratory scale and mass-production (commercial or not possible to extend optimized medium conditions industrial) scale. determined for MSR-1 growth in shake-fask culture directly to mass-production scale fermentor culture [15]. Mass production of magnetosomes During MSR-1 culture, accumulation of excessive nutri- Mass production of magnetosomes for commercial ents and inhibitory components in medium exerts rate- applications remains a challenging task. Cultivation of limiting efects on cell growth. According to Liebig’s Law magnetotactic bacteria (MSR-1, AMB-1, MS-1) is dif- of the Minimum, biomass in a given system is typically fcult because of their diverse metabolisms, although restricted by the amount of one particular nutrient, even several types of culture media have been developed when other nutrients are present in excess [16]. for high magnetosome yield [14]. MSR-1 has been uti- A nutrient-balanced feeding strategy can reduce the lized extensively as a model microorganism for studies inhibitory efect of excessive amount of nutrients in of magnetosome formation. Growth conditions afect medium. In this strategy, accumulation of Na + and Cl − the physical properties of magnetosomes synthesized by ions is reduced by replacement of carbon and nitrogen MSR-1. In particular, depending on the bacterial growth sources. In fed-batch culture, accumulation of Na+ and phase (either at the logarithmic phase or at the stationary Cl − ions decreases osmotic potential and consequently

Fig. 1 Culture media used for cultivation of M. gryphiswaldense strain MSR-1, and factors that afect MSR-1 growth [15] Basit et al. Microb Cell Fact (2020) 19:197 Page 3 of 11

inhibits cell growth. Even a low NaCl concentration MSR-1 cell growth and magnetosome formation are (40 mM [2.34 g L− 1]) inhibited cell growth [17]. Tus, high when sodium lactate is used as carbon source. On a nutrient-balanced feeding strategy can signifcantly the other hand, low sodium lactate concentration is enhance growth rate. needed to maintain low dissolved oxygen concentration Liu et al. [15], established a “chemostat culture” tech- for rapid cell growth and magnetosome formation [15]. nique for MSR-1 cultivation based on pH-stat feeding Maintaining sodium lactate concentration in mass pro- to maintain consistency of nitrogen, carbon, and iron duction scale-up is difcult, and specifc feeding strate- concentrations using several organic acids. Microaero- gies are therefore required in the laboratory. NH4Cl has bic conditions were applied for MSR-1 cultivation in a been shown to be a better nitrogen source than NaNO3 fed-batch autofermentor system. A nutrient solution [15, 17]. containing (per liter) ferric citrate (4.2 g), lactic acid Zhang et al. [17], achieved maximal magnetosome yield (52.6 g), sodium lactate (129 g), and NH4Cl (54.9 g) was in MSR-1 using a semi-continuous culture strategy. Opti- used for pH-stat feeding. High values of magnetosome mized fask medium was used in 7.5- and 42-L autofer- − 1 yield (83.23 ± 5.36 mg L ) and cell growth (55.49 mg mentors, nutrient-balanced feeding strategy was applied, L− 1 day − 1) were achieved at low sodium lactate level and carbon and nitrogen sources were replaced to reduce (Table 1). Chemostat culture technique efciently pro- accumulation of Na+ and Cl− ions. Osmotic potential motes magnetosome yield and cell growth with low was decreased by Na+ and Cl− ion accumulation, thereby time and energy cost. Cytotoxic efects were observed inhibiting magnetosome yield and cell growth. We for excessive dissolved oxygen concentration (≥ 20 ppb) achieved maximal values in fed-batch culture of magne- and presence of lactic acid in medium. Artifcial control tosome yield 356.52 mg L− 1 and cell growth 9.16 g L− 1 strategies for autofermentor systems must be adjusted [17] (Table 1). in regard to physiological condition of cells. Similarly, Yang et al. [19], cultured AMB-1 cells in magnetic Fernández-Castané et al. [18], demonstrated the pH- spirillum growth medium (MSGM) enriched with stat fed-batch growth strategy. In this strategy various L-cysteine, yeast extract, and polypeptone. In this sys- concentrations of the lactic acid (carbon source) and tem, L-cysteine enhanced cell growth and reduced lag sodium nitrate (electron acceptor) were applied in the phase, resulting in high magnetosome production. Te feed. Growth conditions and intracellular iron concen- addition of only yeast extract and polypeptone results tration were optimized according to the biomass con- in slightly production of magnetosomes. Yeast extract centration. Te highest biomass concentration reached displays no signifcant efect in magnetosome produc- to OD565 nm = 15.50 [18]. tion, whereas polypeptone only increases the fnal cell density [20]. Te reason for improved production of

Table 1 Yield production of magnetosomes by magnetotaic bacteria and their conditions Strains Growth Media pH and Temperature Dissolved oxygen Yield production References

1 ME-1 OFM 7.0 and 30 °C 0.5 ppm 120 mg L− [6] 1 MS-1 MSGM PEG 7.0 and 28 °C 1% 21.8 mg L − [11] + MSR-1 OFM 6.8 and 30 °C 0 ppm 83.23 5.36 [15] ± 1 mg L− 1 MSR-1 OFM 6.9 and 30 °C 0–1 ppm 225.53 mg L− [17] (feed batch) 1 MSR-1 OFM 6.9 and 30 °C 0–1 ppm 168.3 mg L− [17] (semi-continuous) (dry weight basis) 1 AMB-1 MSGM 6.7 and 25 °C – 7.5 mg L− [19] 1 MSR-1 OCM 6.8 and 30 °C 5–10 ppm 186.67 mg L− [23] 1 AMB-1 LSM 7.0 and 28 °C 0.25 mbar 3.3 mg L− [25] 1 MSR-1 LSM 7.0 and 28 °C 0.25 mbar 6.3 mg L− [25] 1 MSR-1 OFM 7.0 and 30 °C 0.5% 58.4 6.4 mg L− [31] ± 1 AMB-1 MSGM 6.7 and 26 °C – 4.5 mg L− [90] MSR-1 OGM 6.8 and 30 °C > 1 ppm 26 3 [91] Magnetosomes± no. OFM optimized fask medium, OCM optimized culture medium, MSGM Magnetic Spirillum growth medium, LSM large scale medium, OGM optimized growth medium, PEG polyethylene glycol Basit et al. Microb Cell Fact (2020) 19:197 Page 4 of 11

magnetosomes by L-cysteine is unknown, however, it or lowest concentration of impurities other than iron, is assumed that magnetosome production is not asso- paves the way towards medical applications. ciated with lower redox potentials in the presence of L-cysteine [20]. Moreover, AMB-1 can grow without Dissolved oxygen concentration available amino acids and L-cysteine synthesis path- Magnetosome biosynthesis requires microaerobic or ways in AMB-1 may be not efcient or related to cell anoxic conditions. Low dissolved oxygen level signif- growth. Terefore, AMB-1 may directly use L-cysteine cantly afects cell growth because high-density culture instead of having to synthesize it for facilitating cell requires high dissolved oxygen to obtain desired mag- growth [20]. netosome yield. On the other hand, increased dissolved Ke et al. [6], cultured Magnetospirillum sp. ME-1 in oxygen may increase MSR-1 density in culture medium growth medium enriched with sodium acetate, sodium but inhibit magnetosome formation [15]. A confict thus succinate, yeast extract, MgSO 4, NH4Cl and ferric citrate. exists between magnetosome formation and cell growth, ME-1 utilizes carbon source for growth such as succinate, making it difcult to simultaneously achieve high MSR-1 fumarate, oxaloacetate, pyruvate, acetate, lactate, malate cell density and high magnetosome yield. Tis confict and peptone. In addition, ME-1 can grow in the absence may be resolved somewhat by controlling dissolved oxy- of nitrogen source, however, NH 4Cl or NaNO 3 sup- gen to an optimal level through adjustment of cell growth plementation enhance the ME-1 growth. ME-1 exhibit rate. Jajan et al. [23], reported the reduced iron uptake urease and oxidase activity, suggesting the capability of and magnetosomes production at the dissolved oxygen aerobic growth, however, aerobic condition inhibits the level of above 5–10 ppm. However, when dissolved oxy- magnetosome formation in ME-1 [6]. Fed-batch fer- gen was lower than 5–10 ppm, iron uptake rate and pro- mentation of ME-1 was optimized at a constant level of duction of magnetosomes was increased, which probably pH 6.8 in a 10-L fermenter based on pH-static feeding, due to the slow growth of bacteria [23]. Sun et al. [24], while supplying the carbon, nitrogen and iron sources for established mass culture of MSR-1 for enhanced magne- large-scale production [6] (Table 1). tosome production in a 42-L fermentor, with optimized Despite the high yield of magnetosomes, such devel- fask medium, by applying strict microaerobic conditions oped methods for the growth of magnetotactic bacteria (near-zero dissolved oxygen concentration) and using fer- contains toxic components in growth medium. Tese ric citrate and sodium lactate as iron and carbon sources components include carcinogenic, mutagenic and repro- in medium. Tis strategy was efective for yield cultiva- toxic chemicals, heavy metals, chelating agents and un tion of magnetosomes because cell growth was regulated characterized animal derived ingredients such as yeast at low dissolved oxygen concentration, resulting in high extract [21]. Tere is a great need to obtain large scale magnetosome yield. production of pure magnetosomes with lowest possible AMB-1 is a facultative anaerobic magnetotactic bac- amount of such impurities or toxic components (other terium that transfers electrons via two respiratory path- metals than iron). Terefore, Berny et al. [22], developed ways. Under aerobic growth conditions, AMB-1 utilizes a minimal growth medium for magnetosomes produc- oxygen as electron acceptor and neither promotes nor tion with less or devoid of toxic components, and hav- inhibits formation of magnetic particles. In an alternative ing similar magnetosome properties as those obtained pathway, AMB-1 uses nitrate as electron acceptor and in the best reported growth conditions by Zhang et al. therefore requires low media redox potentials, which are [17]. Firstly, magnetotactic bacteria were amplifed in conducive to magnetosome formation. In a study by Yang pre-growth medium [containing sodium lactate (2.6 g), et al. [20], magnetosome production rate was high under NH4Cl (0.4 g), MgSO4·7H2O (0.1 g), K2HPO4 (0.1 g), low dissolved oxygen concentration in liquid phase. thiamin HCl (40 µg), ME6 (0.5 mL)] without produc- When dissolved oxygen concentration in liquid phase ing magnetosomes [22]. In second step, magnetotac- exceeded a certain level (0.20 ppm), the respiratory path- tic bacteria were then fed with an iron rich fed-batch way shifted to aerobic growth, leading to reduced magne- medium containing [sodium lactate (1.3 g), NH 4Cl tosome production. (0.2 g), MgSO4·7H2O (0.03 g), K2HPO4 (0.03 g), thiamin Dissolved oxygen concentration is strongly afected by HCl (27 µg), of ME6 (0.08 mL)] to allow magnetosome air fow rate and stirring rate. When dissolved oxygen synthesis [22]. After 50 h of growth, biomass concentra- during initial growth phase is raised by increasing air tion reached to OD 565 nm = 8 and yield magnetosomes fow and stirring rates, magnetosome yield remains low production of about 10 mg/L of growth medium. A sig- until dissolved oxygen declines to an undetectable level. nifcant reduction/disappearance in magnetosome com- To overcome this phenomenon in MSR-1 cultivation, position of Zn, Mn, Ba, and Al were observed [22]. Tis dissolved oxygen must be enhanced to an optimal level new strategy for the magnetosomes production without by stirring, and cells then allowed to reduce dissolved Basit et al. Microb Cell Fact (2020) 19:197 Page 5 of 11

oxygen through respiration, until reaching the level opti- in culture medium. Jajan et al. [23], showed that fer- mal for magnetosome formation. High magnetosome rous sulfate was a better source of iron than ferric qui- production was achieved by optimizing/ adjusting air nate and ferric citrate for M. gryphiswaldense [23]. In a fow and stirring rates [15]. During initial culture phase, study of AMB-1, Yang et al. [19], used various ferrous dissolved oxygen was reduced by maintaining these rates sulfate and ferric chelates as iron sources, and com- respectively at 1 L min − 1 and 200 rpm min − 1. During pared their efects. Magnetosome production was sig- later culture phase, dissolved oxygen was increased by nifcantly enhanced by ferric gallate and sulfate, and was adjusting air fow rate to 2 L min − 1 at 20 h and stirring also afected by other iron source (ferric quinate, ferric rate to 300 rpm min − 1 at 28 h. Under these conditions, malate), and by iron uptake rate. cells grew rapidly, dissolved oxygen become undetect- Ferric ion (Fe 3+) is taken up during dynamic cell able at 12 h, and cell density reached OD 565 nm = 12.3 at growth, and the amount taken up is correlated with 36 h. Sodium lactate and ferric citrate concentrations magnetosome formation when dissolved oxygen level were controlled respectively in the ranges 3–6 mmol L − 1 in medium is undetectable. Magnetosome formation and 70–110 µmol L− 1 during the process. High values requires micromolar iron concentration and microoxic − 1 of magnetosome yield (83.23 ± 5.36 mg L ) and pro- conditions [27]. MSR-1 cells are nonmagnetic under oxic ductivity (55.49 mg L− 1 day − 1) were thus attained [15] conditions, but begin to produce magnetite when dis- (Table 1). solved oxygen concentration falls below a threshold value In ME-1 cultivation, dissolved oxygen was controlled to (20 mbar or undetectable). In log phase of cell growth, enhance the magnetosome production at a constant level ferric ion is taken up rapidly, and its absorption rate is of 0.5% by coupling to the air-fow rate and stirring rate. > 80% and correlated with magnetosome formation [15]. During fed-batch fermentation, a stirring rate (in the range of 50–300 rpm) produced a large amount of magnetosomes at constant level of dissolved oxygen (0.5%). Magnetosome synthesis consumes ATP Te resulting cell density and magnetosome yield at 49 h ATP is the universal energy source required for metabo- − 1 were 6.5 (OD565) and 120 mg L (wet weight). Tis strat- lism, molecular transportation, signal transduction, and egy attained high magnetosome yield and productivity, other crucial cell physiological processes. Magnetosome thus indicate that ME-1 has great potential for the large- synthesis requires a large amount of energy, and iron scale production of magnetosomes [6]. uptake depends on ATP availability. NADH provides a Low dissolved oxygen levels have been established proton gradient across the inner mitochondrial mem- empirically in many studies, but without continuous brane for ATP production catalyzed by ATP synthase measurement of dissolved oxygen concentration or def- [28]. NADH/NAD+ ratio increases rapidly following nition of its control in the medium. Heyen and Schüler magnetosome maturation during log phase. [25], established a method for automatic control of low Reducing power increases signifcantly during mag- oxygen tension (pO 2) in MSR-1 culture medium using netosome synthesis; however, excessive reducing power a fermentor system for oxystat operation. pO 2 tension may inhibit magnetosome synthesis and cell growth [17]. was correlated with magnetite formation. Te lowest MSR-1 can consume excessive reducing power through 5 recorded pO2 value (0.25 mbar; 1 bar = 10 Pa) was the polyhydroxybutyrate (PHB) synthesis and hydrogen most favorable for magnetosome formation. Cells grown release [29]. MSR-1 cells contain PHB granules [30]. under oxystat conditions showed signifcantly higher Knockout of PHB synthase gene in MSR-1 resulted in magnetite yield (6.3 mg L− 1 day − 1) (Table 1). ~ 30% increase of magnetosome number [31]. Energy competition thus occurs between PHB and the magne- Ferric ion uptake tosome synthesis process. A mutant of ATPase gene of Iron is required as a cofactor for many enzymes, particu- MSR and MSR-NPHB created by conjugation was used larly those involved in major biological pathways. Specifc as a genetic engineering tool to demonstrate that chlo- iron transport mechanisms in cells provide iron levels ramphenicol acetyltransferase (CAT) promoter increases sufcient for optimal growth. Some bacteria produce downstream expression of ATPase gene. In compari- ferric chelators (termed siderophores) to take up ferric son with MSR-1, MSR-NPHB displayed 35% greater iron (Fe3+). Magnetotactic bacteria synthesize magne- hydrolysis activity, 71% lower PHB accumulation, 56% tosomes composed of magnetite or greigite after fnding greater oxygen consumption, and 40% greater lactate microaerophilic conditions suitable for their growth [26]. consumption. Maximal yield of MSR-NPHB in a 7.5-L − 1 In MSR-1, magnetite is the major component of mag- bioreactor was 58.4 ± 6.4 mg L [31]. Tese fndings netosomes, and magnetosome production is therefore demonstrate that magnetosome yield can be enhanced, not signifcantly afected by ferric citrate concentration and production cost and time reduced, through genetic Basit et al. Microb Cell Fact (2020) 19:197 Page 6 of 11

manipulation of MSR-1 in combination with optimiza- in the homogenizer, and cell extract was pumped into tion/ modifcation of culture and growth media. the MIS. Solution at controlled fow rate was passed through the homogenizer, lysed solution was pumped Superoxide dismutase activity into the MIS, and waste residue was discharged through Magnetosome synthesis is associated with in vitro break- the outlet. Magnetosomes in the MIS were washed with down of H2O2, and with protective efects against H2O2 six bed-volumes of bufer containing various concentra- toxicity in cells. In microorganisms, the enzyme superox- tions of urea, and then washed into the ultrasonic tank by ide dismutase breaks down H 2O2 and superoxide anion ultrasonication process. Urea solution in the ultrasonic − radical (O2 ), both of which have destructive efects on tank was replaced by the bufer, magnetosomes were dis- cell macromolecules [32]. In magnetotactic bacteria, persed by ultrasonication, and bufer and magnetosome superoxide dismutase also reduces oxidative stress dur- solution were simultaneously pumped back into the MIS ing magnetosome formation. H2O2 may form a hydroxyl by operating pumps. After binding of magnetosomes by radical after receiving an electron from ferrous iron magnetic force, six bed-volumes of bufer were pumped (Fe 2+). Hydroxyl radical is the reactive oxygen species into the MIS. Ultrasonication was activated and mag- (ROS) that can damage biomolecules [33]. Yang et al. net removed in the MIS such that magnetosomes were [30], demonstrated that when sufcient dissolved oxygen released and washed back into the ultrasonic tank. Te and nutrients are available in late log phase, magneto- above procedure was repeated three or four times until some formation and maturation are unable to catch up proteins in waste solution were undetectable at OD 280. to cell division rate, leading to dilution of magnetosomes. Te industrial development of magnetosomes has been Reduction of superoxide dismutase activity may thus raised by high yield magnetosome production methods. result from similarity of diluted ROS. Magnetosomes, However, magnetotactic bacterium are Gram-negative as well as artifcial magnetic nanoparticles, participate bacteria, which contains bacterial endotoxin such lipo- in scavenging of ROS [27, 34], and this activity may also polysaccharides [38], that probably efect magnetosomes lead to reduced superoxide dismutase activity. applications. Terefore, several studies revealed the production of sterile or endotoxin free magnetosomes by Rapid large‑scale purifcation of magnetosomes removing of surrounding biological materials from mag- Improved and optimized methods for extraction and netosomes mineral core [38, 39]. Since magnetosome purifcation of magnetosomes are needed, for reasons formation results from the invagination of the bacterial mentioned in Introduction. Magnetosome extraction membrane, the endotoxin concentration at the magne- methods have been described in numerous publications tosome surface might be higher than the tolerated con- [35–37]. However, methods described to date for mag- centration. Terefore, followed by the extraction and netosome purifcation, which involve disruption of cells purifcation of magnetosomes from magnetotactic bacte- by sonication, washing with PBS bufers, ultrasonic bath- ria, organic materials such as endotoxins can be removed ing, and collection of magnetosomes using a magnet with the treatment of diferent detergents (SDS, Triton (Fig. 2A), are not suitable for large-scale systems. Guo X100, phenol and chloroform) at 60°C in the presence et al. [36], developed a rapid, continuous system for large- of sonication [39]. Te obtained uncoated magnetosome scale magnetosome purifcation. Te system involved minerals can be characterized by transmission electron a high-pressure homogenizer, a magnetic isolation sys- microscope (TEM). Te detection and quantifcation tem (MIS), ultrasonic and electro-elution equipment, of endotoxin concentration of uncoated magnetosome and magnetic stirring (Fig. 2B). Te MIS consisted of an minerals can further be estimated by limulus amebocyte ultrasonic tank, a permanent magnet, and a magnetic lysate (LAL) assay. Deduced endotoxin concentration isolation column. Te magnetic isolation column was a can be further verifed by in vivo experiments. Hamdous plastic cylinder flled with iron beads (diameter 4–6 mm), et al. [38], in their study, examined the endotoxin con- with four interfaces (inlet or outlet) at the two ends. Te centrations by administering the purifed magnetosome two interfaces at the bottom end carried bufer and solu- suspension in the rabbits’ ear. Te induction of any fever tion containing cells/ magnetosomes. Te two interfaces among rabbits was indicated the pyrogenic or non-pyro- at the top end carried bufer and discharged wastewater. genic suspensions. Te bufers used in the MIS were sterilized by heating at 121°C for 20 min, and the magnetic isolation column Applications was cleaned before use by pumping through a 75% etha- Magnetosomes are of considerable research and prac- nol solution (v/v). Te high-pressure homogenizer and tical interest because of unique features such as nar- MIS were prewetted with one bed-volume of PBS bufer. row size distribution, uniform morphology, high Culture solution mixed with bufer was used to lyse cells chemical purity, low toxicity, and membrane envelope Basit et al. Microb Cell Fact (2020) 19:197 Page 7 of 11

Fig. 2 Two systems for extraction and purifcation of magnetosomes (see text)

[37]. Applications of magnetosomes refect their dis- Fig. 3. Recent studies demonstrated that purifed, steri- tribution, ferrimagnetic properties, nanoscale size, lized MSR-1 magnetosomes are nontoxic to cancer membrane-bound structure, and dispersal ability. cells in vitro, suggesting their potential application as Applications of magnetosomes are summarized in drug carriers for clinical treatment of cancer and other Basit et al. Microb Cell Fact (2020) 19:197 Page 8 of 11

Fig. 3 Schematic representation of magnetosomes applications diseases [35, 40–42]. Magnetosome membranes con- dendritic cells and single nucleotide polymorphism tain proteins and active groups that can function as (SNP) [59, 60], detection of pathogenic bacteria, viruses binding sites. Synthetic magnetic nanoparticles utilize [61–63] and other immunodetection [64, 65], discrimi- specifc active surface groups, through either chemical nation or recovery of DNA and mRNA [59, 66–68], coupling or physical adsorption. Because of the pres- adsorption and mineralization of metals [69–74] and ence of a natural phospholipid membrane, coupling of known as the powerful tool for chip-based whole-cell magnetosomes with active substances such as chemo- biosensors [75]. “Surface display system” of magneto- therapeutic drugs [37, 43] and peptides [40] is of great somes used in immunoassay, enzyme reaction, and cell interest in biomedicine and drug delivery [44–50]. separation [76–87]. Magnetosomes can also be used as Magnetosomes wrapped with polyethyleneimine (PEI) an efective model system to study CDF related II type can be used in gene delivery and cancer treatment diabetes [88]. Magnetotactic bacteria used to produce [51–53]. Magnetosomes extracted from magnetotac- electricity and magnetic feld induced rotation of mag- tic bacteria can be used as a magneto-thermal therapy netosome chains in silicifed MTB [78, 89]. Tus, avail- for cancer treatment [54–56]. Other practical appli- ability of high-quality magnetosomes will signifcantly cations of magnetosomes includes as magnetic reso- increase potential applications, and it is therefore nance imaging (MRI) probe or modifed with protein highly desirable and important to establish and develop for tumor detection [57, 58], the tracking of stem cells, rapid, simple, relatively inexpensive processes for yield cultivation of magnetosomes. Basit et al. Microb Cell Fact (2020) 19:197 Page 9 of 11

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