Clostridium limosum
1. Classification
Clostridium limosum or Hathewaya limosa
a. Higher order taxa
Bacteria; Firmicutes; Clostridia; Eubacteriales; Clostridiaceae; Hathewaya [1]
2. Description and significance
Clostridium limosum is a Gram-positive bacterium that has large, straight, rod-shaped cells. C. limosum is found in soil, but it has also been known to infect humans and animals such as cattle, farm minks, and chickens [2] [3]. The pathogenic properties are still a source of current research as the route of transmission is not fully understood, with a special focus on potential foodborne illness, because of the clinical disease implications in livestock [2][4]. C. limosum is a close relative of Clostridium botulinum, the bacteria that cause the illness of botulism via a toxin that attacks the nervous system [2]. However, treatment for botulism does not successfully treat a C. limosum infection, making the comparison between the two a source of interest [2]. There is previous evidence of human infection in lung abscesses as a part of a mixed infection [5][6]. C. limosum infections within humans are known to be exceedingly rare, but carry a large risk of future complications and occasionally death [6].
3. Genome structure
The complete genome was sequenced from a Clostridium limosum strain that was isolated from the intestinal tissues of a group of cows that suffered from a potential blackleg outbreak in Germany, 2014 [4]. The genome of this strain (14S0207) consists of a circular chromosome and three plasmids of varying sizes and numbers of genes. The circular chromosome is 2.95Mb, containing 33 rRNAs, 92 tRNAs, four other RNAs, 2718 genes, and 62 pseudogenes [4]. A streptolysin-related gene cluster found in the chromosome codes for a streptolysin S homolog (which encodes 52 amino acids), a bacteriocin, and a virulence factor that can cause hemolytic or cytolytic activity in the infected host cells. On the other hand, the first plasmid (0.14 Mb) contains 125 proteins, 132 genes, and 7 pseudogenes; the second plasmid (0.04 Mb) contains 54 proteins, 58 genes, and 4 pseudogenes; the third plasmid (0.03 Mb) contains 30 proteins, 31 genes, and one pseudogene [4]. None of the three different plasmids carries any rRNA or tRNA information. Each of the four components of C. limosum has similar guanine-cytosine content and the overall guanine-cytosine content for the genome is 24.0 mol% [4]. There are three highly conserved proteins within the genus of Clostridium that contain three highly conserved indels: a four-amino-acid insert in DNA gyrase, one- amino-acid deletion in ATP synthase beta subunit, and a one-amino-acid insert in ribosomal protein S2 [3]. A C3 transferase exoenzyme obtained from a human pathogenic strain of C. limosum catalyzes ADP-ribosylation, a mechanism that the bacteria use to attack eukaryotic cells and modify their regulatory proteins[5].
4. Cell structure
On blood agar plates, isolated colonies of Clostridium limosum have circular and irregular shapes with smooth, scalloped, or wavy edges [3]. The surface diameter of the colonies ranges from one to four millimeters, and both raised and convex elevations are present. Colonies of C. limosum cells have a grey color, and some are shiny while others are dull under the reflection of light[3]. The texture of the bacterial colonies is generally smooth[3]. C. limosum is a Gram-positive bacterium. Under the compound microscope, cells of C. limosum appear as straight rods, 0.6-1.6 μm (width) × 1.7-16 μm (length) in size. They either arrange singularly, in pairs, or chains [3]. The cell wall contains amino acids such as meso-Diaminopimelic acid (meso-DAP), glutamic acid, and alanine, while glucose and galactose are its major carbohydrate components. The cellular lipids of C. limosum are mainly composed of straight, saturated C16, C14, and C12 fatty acids[3]. Oval-shaped spores are formed at the center or toward one end of the cell. Motility is varied among the cells of C. limosum, which can move via peritrichous flagella, lash-like organelles that are uniformly distributed on the cell body to enhance movement [3].
5. Metabolic processes
Clostridium limosum is catalase-negative and is hemolytic when grown on blood agar plates[2]. It forms oval, subterminal endospores that are located between the middle and the end of the bacterial cell[3]. In a broth culture containing acetate, formate, succinate, and or lactate, C. limosum can ferment an indole and phenol from tryptophan and tyrosine and can also ferment histidine[3]. When tested for indole production and hydrogen sulfide production, the bacterium was tested positive for both[7]. C. limosum can produce lecithinase, but it is incapable of fermenting a variety of sugars[8]. With an epimerization mechanism, C. limosum can transform primary bile acids to urso- and 7-keto-bile acids. Over time and under oxygen exposure, C. limosum can oxidize urso-bile acids back to 7-keto bile acids[8]. Some strains are incapable of this mechanism, which gives rise to a strain-to-strain variation for this characteristic[8].
6. Ecology
Clostridium limosum is mesophilic and grows best in an anaerobic atmosphere from 30°C to 45°C depending on the individual strain[3]. During heat testing, C. limosum was able to resist heat at 80 degrees Celsius for 10 minutes. Cultures of C. limosum were grown in different pH mediums. Although growth occurred significantly slow, the bacteria were able to grow at a pH up to 9 [7]. Its optimal growth occurs at a pH of 7. C. limosum shows growth in NaCl solutions but cannot survive past 6.5% NaCl in solution[7]. C. limosum appears to be sensitive to most antibiotics. These antibiotics include chloramphenicol, erythromycin, penicillin, and tetracycline. In contrast, the bacteria showed high levels of resistance to clindamycin[7]. Isolates of C. limosum are obtained from infections in animals or soil samples around the world[8]. A strain of C. limosum has been isolated from African mud samples[8], and C. limosum has also been found in the uterus of pregnant farmed mink in Finland [2]. C. limosum is classified as mesophilic, while the optimum temperature for growth can vary, allowing it to survive in many different ecological conditions.
7. Pathology
While Clostridium limosum is present in many hosts without causing disease, it has the potential to cause infection. Only one case of illness in humans has ever been documented with C. limosum as the primary disease-causing agent. The infection occurred in a prosthetic heart valve[6]. C. limosum is more likely to manifest in other animal hosts and usually occurs in mixed bacterial infections[7]. Intramuscular inoculation of C. limosum into guinea pigs can lead to a loss of turgor and ultimately the degradation of their muscle cells, due to collagenase and lecithinase activity in host muscle tissues. Lesions were noted on the outside of the animal's skin[7]. C. limosum is also capable of producing an exoenzyme that catalyzes ADP-ribosylation, a process that selectively modifies the small GTP-binding Rho proteins in the membranes of human platelet cells. This mechanism, which can also be performed by C. botulinum-- a close relative of C. limosum, is used to attack eukaryotic cells and modify their regulatory proteins[5] [9]. Additionally, the cell-permeable C3 exoenzyme that the bacterium produces has been known to internalize and intoxicate human dendritic cells and change their morphology through its action on GTPase[10]. This can lead to repressed macrophage signaling, reduced host defense, and ultimately cell death[11]. It is possible to detect the toxin's presence based on changes in the organization of the host cell’s cytoskeleton[11].
8. Current Research
In the Spring of 2013, it was determined that Clostridium limosum was responsible for a metritis outbreak in a Finish mink farm. In this instance, metritis was only observed in pregnant females and the infection primarily remained within the uterus[2]. A high-quality genome sequence of C. limosum was recovered from a cow that died from suspected blackleg in 2014[4]. These two cases continue to be helpful to research, as full details of pathogenic properties and transmission of C. limosum are unknown. Most recently, in 2019, a man with prosthetic valve endocarditis was the first person infected with C. limosum in which C. limosum was the sole pathogen causing the infection[6]. Currently, there is debate about if the genus Clostridium is too broad. The G+C content of the chromosomal DNA varies between 21 to 54% within the genus, which is considered too large of a range for any single genus [3].
9. References
Edited by [Julie Birch, Michael Breen, Ayla Celik, Zikeng Huang, and Julianne Leopard], student of Jennifer Bhatnagar for BI 311 General Microbiology, 2020, Boston University.
