Antibacterial Secondary Metabolites of the Cyanobacterium Lyngbya Morphotype

Antibacterial Secondary Metabolites of the Cyanobacterium Lyngbya Morphotype: Comparison

Please note this is a comparison between Version 1 by Lamiaa Ahmed Shaala and Version 3 by Dean Liu.

Cyanobacteria ascribed to the genus Lyngbya (Family Oscillatoriaceae) represent a potential therapeutic gold mine of chemically and biologically diverse natural products that exhibit a wide array of biological properties. Phylogenetic analyses have established the Lyngbya ‘morpho-type’ as a highly polyphyletic group and have resulted in taxonomic revision and description of an additional six new cyanobacterial genera in the same family to date. Among the most prolific marine cyanobacterial producers of biologically active compounds are the species Moorena producens (previously L. majuscula, then Moorea producens), M. bouillonii (previously L. bouillonii), and L. confervoides. Over the years, compounding evidence from in vitro and in vivo studies in support of the significant pharmaceutical potential of ‘Lyngbya’-derived natural products has made the Lyngbya morphotype a significant target for biomedical research and novel drug leads development. RThis comprehesearchers concludednsive review covers compounds with reported anti-infective activities through 2022 from the Lyngbya morphotype, including new genera arising from recent phylogenetic re-classification. So far, 72 anti-infective secondary metabolites have been isolated from various Dapis, Lyngbya, Moorea, and Okeania species.

marine cyanobacteria
Lyngbya morphotype
secondary metabolites
Antibacterial

1. Introduction

For more than three billion years, cyanobacteria inhabited the earth, representing one of the eldest known organisms ^[1]. Cyanobacteria are diverse in terms of their physiology, metabolism and morphology. They inhabit all environments worldwide, including freshwater, marine environment and extreme habitats ^[2]. The production of highly potent/toxic secondary metabolites is considered as an evolutionary strategy for cyanobacteria to survive from planktivores or other ecological competitors ^[3][4]. It was noticed that a significant portion of the cyanobacterial secondary metabolites in the literature possessed cytotoxic activity ^[5]. Certainly, cytotoxicity is still today a noticeable activity of cyanobacteria-derived compounds.

Specimens characterized by relatively large filaments of discoid cells within a distinct sheath and lacking nitrogen-fixing heterocyst cells have routinely been assigned to the genus Lyngbya (Family Oscillatoriaceae, Order Oscillatoriales). This traditional morphology-based taxonomic identification has underestimated the biological diversity of filamentous marine cyanobacteria ^[6] leading to more than 500 marine-derived compounds being ascribed to the single genus Lyngbya. While this Lyngbya ‘morpho-type’ has evolved to additionally include the new genera Dapis ^[7], Limnoraphis ^[8], Moorena (previously Moorea) ^[9], Microseira ^[10] and Okeania ^[11], and further taxonomic revisions are in progress, it remains a group with tremendous therapeutic potential.

2. Compounds with Antibacterial Activities

Among the diverse bioactivities that Lyngbya secondary metabolites have displayed is the activity against bacteria. In 1979, malyngolide (1) (Figure 1), a δ-lactone was reported from Hawaiian Lyngbya majuscula in Kahala Beach, showed effective antibacterial activity against Mycobacterium smegmatis and Streptococcus pyogenes and was less active against Staphylococcus aureus and Bacillus subtilis, and inactive towards Enterobacter aerogenes, Escherichia coli, Pseudomonas aeruginosa, Salmonella enteritidis, and Staphylococcus marcescens ^[12].

Figure 1. Chemical structures of compounds 1–3.

In 1987, the fatty acid (-)-7(S)-methoxytetradec-4(E)-enoate (lyngbic acid) (2) (Figure 1), was purified from Moorea producens collected at the Red Sea, near Jeddah, Saudi Arabia ^[13], displayed antibacterial activity against S. aureus and B. subtilis ^[13]. The related amide of lyngbic acid, malyngamide D acetate (3) (Figure 1), which were isolated from Caribbean L. majuscula in Isla Guayacan, Puerto Rico in 1987, displayed slight activity against S. aureus ^[14]. In 2001, the cyclic depsipeptides pitipeptolides A (4) and B (5) (Figure 2) are reported from L. majuscula collected in Piti Bomb Holes, Guam ^[15]. The compounds displayed moderate activity against Mycobacterium tuberculosis strains (ATCC 25177 and ATCC 35818) in the antimycobacterial diffusion susceptibility assay. Pitipeptolide A (4) gave a diameter of growth inhibition zone for ATCC 25177 strain equivalent to 25 and 10 mm, and for ATCC 35818 strain equivalent to 15 and 9 mm upon treatment with 100 and 25 µg, respectively. Pitipeptolide B (5) gave a diameter of growth inhibition zone for ATCC 25177 strain equivalent to 30 and 15 mm, and for ATCC 35818 strain equivalent to 15 and 10 mm upon treatment with 100 and 25 µg, respectively. For comparison, treatments with 25, 5 and 1 µg of streptomycin resulted in superior activity, giving diameters of 50, 15 and 0 mm, respectively, for ATCC 25177 strain, and 55, 33 and 10 mm, respectively, for ATCC 35818 ^[15].

Figure 2. Chemical structures of compounds 4–9.

Ten years later, in 2011, pitipeptolides C-F (6–9) (Figure 2) are reported from L. majuscula in Piti Bomb Holes, Guam. The study showed that pitipeptolide F (9) was the most potent compound in the antimycobacterial disc diffusion assay (M. tuberculosis ATCC 25177 strain) ^[16]. Treatment with pitipeptolides A-F (4–9) resulted in inhibition zones of 28, 30, 26, 10, 21 and 40 mm, respectively, at 100 µg, 23, 24, 21, 0, 15 and 30 mm, respectively, at 50 µg, and 9, 14, 18, 0, 0 and 10 mm, respectively, at 10 µg. For comparison, streptomycin gave 40, 30 and 0 mm inhibition zone upon using 10, 5 and 1 µg treatment, respectively ^[16]. SAR studies revealed that the N-methylation in the Phe unit is essential for both cytotoxic and antibacterial activities, whereas the π system in the fatty acid unit was found to be one of the important structural features for the cytotoxic activity in mammalian cells, but it was not required for antibacterial activity. Furthermore, decreasing the hydrophobicity of certain units (2-Hydroxy 3-methyl pentanoic acid (Hmpa) → 2-Hydroxy isovaleric acid (Hiva) and Ile → Val) caused a reduction in the anticancer activity (as seen with pitipeptolides E and F), while on the other hand resulted in an increase in antimycobacterial potency (particularly pitipeptolide F) ^[16]. Pitiprolamide (10) (Figure 3), a dolastatin 16 analog and a proline rich cyclic depsipeptide was purified in 20111 from the same Guamanian cyanobacterium Lyngbya majuscula collected at Piti Bomb Holes, displayed weak antimycobacterial effect against M. tuberculosis (ATCC 25177 strain) starting at 50 μg in a disk diffusion assay. The compound displayed zone of inhibition of 23, 13 and 0 mm after 100, 50 and 10 µg treatment. Also, the compound exerted weak antibacterial activity against Bacillus cereus (ATCC 10987 strain) starting at 1 μM in a microtiter plate-based assay with an approximate IC₅₀ value of 70 μM and lacked the activity against S. aureus and P. aeruginosa ^[17].

Figure 3. Chemical structure of compound 10.

Table 1 that purified in 2013 from the cyanobacterium M. producens collected at the Red Sea, near Jeddah, Saudi Arabia, significantly inhibited the growth of M. tuberculosis H₃₇Rv in vitro (65% inhibition) at a concentration of 12.5 μg/mL, while the chlorinated lipopetides malyngamides A (11), B (12) and 4 (13) (Figure 4) (obtained from the same cyanobacterial collection) displayed much weaker antimycobacterial activity at the same tested concentration, which was deemed as ineffective (18, 10 and 17% inhibition, respectively) ^[13]. This result suggests the importance of a terminal free carboxylic acid moiety for the antimycobacterial effect.

Figure 4. Chemical structures of compounds 11–13.

Table 12. Compounds with reported antibacterial activities.

Compound	Source Organism	Collection Site	Targeted Bacteria	MIC/Inhibition Zone/IC₅₀	Reference
Malyngolide (1)	L. majuscula	Hawaii, USA	M. smegmatis, S. pyogenes, S. aureus and B. subtilis	More active against M. smegmatis and S. pyogenes than S. aureus and B. subtilis
Malyngolide (1)	L. majuscula	^[	Hawaii, USA	M. smegmatis, S. pyogenes, S. aureus and B. subtilis	^12]
More active against	M. smegmatis	and	S. pyogenes	than S. aureus and B. subtilis	^[12]	Lyngbic acid (2)	M. producens	Red Sea	M. tuberculosis H₃₇Rv	65% inhibition at 12.5 μg/mL	^[13]
Lyngbic acid (
Lyngbic acid (2)	M. producens	Red Sea	M. tuberculosis H₃₇Rv	65% inhibition at 12.5 μg/mL	^[13]	2)	L. majuscula	Caribbean region	S. aureus and B. subtilis	Antibacterial activity	^[14]
Lyngbic acid (2)	L. majuscula	Caribbean region	S. aureus and B. subtilis	Antibacterial activity	^[14]	Malyngamide D acetate (3)	L. majuscula	Caribbean region	S. aureus	Slight activity	^[14]
Malyngamide D acetate (3)	L. majuscula	Caribbean region	S. aureus	Slight activity	^[14]	Pitipeptolide A (4)	L. majuscula	Guam	M. tuberculosis ATCC 25177
Pitipeptolide A (4)	25 mm at 100 µg	L. majuscula	Guam	M. tuberculosis ATCC 25177 10 mm at 25 µg	^[15]
25 mm at 100 µg		10 mm at 25 µg	^[	¹⁵	^]	Pitipeptolide A (4)	L. majuscula	Guam	M. tuberculosis ATCC 35818	15 mm at 100 µg 9 mm at 25 µg	^[15]
Pitipeptolide B (
Pitipeptolide A (4)	L. majuscula	Guam	M. tuberculosis ATCC 35818	15 mm at 100 µg 9 mm at 25 µg	^[15]	5)	L. majuscula
Pitipeptolide B (5	Guam	)	L. majusculaM. tuberculosis ATCC 25177	Guam30 mm at 100 µg	M. tuberculosis ATCC 2517715 mm at 25 µg	30 mm at 100 µg ^[15]
15 mm at 25 µg	^[	¹⁵	^]	Pitipeptolide B (5)	L. majuscula	Guam	M. tuberculosis ATCC 35818	15 mm at 100 µg
Pitipeptolide B (5)	10 mm at 25 µg	L. majuscula	^[	Guam	M. tuberculosis ATCC 35818^15]
15 mm at 100 µg		10 mm at 25 µg	^[	¹⁵	^]	Pitipeptolide A (4)	L. majuscula	Guam
Pitipeptolide A (4)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis ATCC 2517728 mm at 100 µg 23 mm at 50 µg 9 mm at 10 µg	28 mm at 100 µg 23 mm at 50 µg 9 mm at 10 µg^[16]
^[	¹⁶	^]	Pitipeptolide B (5)	L. majuscula	Guam
Pitipeptolide B (5)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis30 mm at 100 µg 24 mm at 50 µg 14 mm at 10 µg	^[16]
ATCC 25177	30 mm at 100 µg		24 mm at 50 µg	14 mm at 10 µg	^[16]	Pitipeptolide C (6)	L. majuscula	Guam
Pitipeptolide C (6)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis ATCC 2517726 mm at 100 µg 21 mm at 50 µg 18 mm at 10 µg	26 mm at 100 µg 21 mm at 50 µg 18 mm at 10 µg^[16]
^[	¹⁶	^]	Pitipeptolide D (7)	L. majuscula	Guam
Pitipeptolide D (7)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis10 mm at 100 µg 0 mm at 50 µg 0 mm at 10 µg	^[16]
ATCC 25177	10 mm at 100 µg		0 mm at 50 µg	0 mm at 10 µg	^[16]	Pitipeptolide E (8)	L. majuscula	Guam
Pitipeptolide E (8)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis ATCC 2517721 mm at 100 µg 15 mm at 50 µg 0 mm at 10 µg	21 mm at 100 µg 15 mm at 50 µg 0 mm at 10 µg^[16]
^[	¹⁶	^]	Pitipeptolide F (9)	L. majuscula	Guam
Pitipeptolide F (9)	M	L. majuscula	. tuberculosis ATCC 25177	Guam	M. tuberculosis40 mm at 100 µg 30 mm at 50 µg 10 mm at 10 µg	^[16]
ATCC 25177	40 mm at 100 µg		30 mm at 50 µg	10 mm at 10 µg	^[16]	Pitiprolamide (10)	L. majuscula	Guam	M. tuberculosis ATCC 25177
Pitiprolamide (10)	23 mm at 100 µg		L. majuscula	13 mm at 50 µg	Guam	M. tuberculosis ATCC 251770 mm at 10 µg	23 mm at 100 µg 13 mm at 50 µg^[17]
	0 mm at 10 µg	^[	¹⁷	^]	Pitiprolamide (10)	L. majuscula	Guam	B. cereus ATCC 10987
Pitiprolamide (10)	IC	₅₀	= 70 μM at 1 μM	L. majuscula	Guam	B. cereus ATCC 10987	IC₅₀ = 70 μM at 1 μM^[17]
^[	¹⁷	^]	Mixture of lyngbyazothrins A and B (14 and 15)	Lyngbya sp.	Germany (Culture)	M. flaVus SBUG 16
Mixture of lyngbyazothrins A and B (14 and	8 mm at 100 μg/disk	^[	¹⁸	^]
15)	Lyngbya sp.	Germany (Culture)	M. flaVus SBUG 16	8 mm at 100 μg/disk	^[18]	Mixture of lyngbyazothrins C (16) and D (17)
Mixture of lyngbyazothrins C (16) and D (	Lyngbya sp.	Germany (Culture)	B. subtilis SBUG 14 E. coli ATCC 11229 E. coli SBUG 13 P. aeruginosa ATCC 27853 S. marcescens SBUG 9	18 mm at 25 μg/disk 18 mm at 100 μg/disk 15 mm at 100 μg/disk 8 mm at 100 μg/disk 8 mm at 200 μg/disk	^[18]
Tiahuramide A (18)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 27, 33, >50, 35 and 47 μM, respectively	^[19]
Tiahuramide B (19)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 9.4, 8.5, 22, 12 and 29 μM, respectively	^[19]
Tiahuramide C (20)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 6.7, 7.4, 16, 14 and 17 μM, respectively	^[19]

Another group of antimicrobial natural products is the cyclic undecapeptides lyngbyazothrins A and B (14 and 15) and C and D (16 and 17) (Figure 5), which were isolated as binary mixtures from Lyngbya sp. 36.91 SAG (Culture Collection of Algae, Gottingen, Germany) in 2009. The mixture of lyngbyazothrins A and B (14 and 15) showed minimal antibacterial activity against Micrococcus flavus SBUG 16 in the agar diffusion disk (100 μg/disk: 8 mm diameter of inhibition zone). The mixture of lyngbyazothrins C (16) and D (17) showed modest activity against B. subtilis SBUG 14 (25 μg/disk: 18 mm), E. coli ATCC 11229 (100 μg/disk: 18 mm), and E. coli SBUG 13 (100 μg/disk: 15 mm) and low activity against P. aeruginosa ATCC 27853 (100 μg/disk: 8 mm) and Serratia marcescens SBUG 9 (200 μg/disk: 8 mm). When used at the same concentrations, the lyngbyazothrins A and B (14 and 15) mixture lacked activity against the aforementioned strains, which suggests that the linkage of the acyl residue at C-5 of the 3-amino-2,5,7,8-tetrahydroxy-10-methylundecanoic acid (Aound) unit may be responsible for the antimicrobial activity ^[18].

Figure 5. Chemical structures of compounds 14–17.

The intriguing cyclic depsipeptides, tiahuramides A-C (18–20) (Figure 8), are isolated in 2018 from L. majuscula collected at Tiahura sector, Moorea Island in French Polynesia, displayed growth inhibitory activities on opportunistic marine pathogenic bacteria (Aeromonas salmonicida (CIP 103209T strain), Vibrio anguillarum (CIP 63.36T), and Shewanella baltica (CIP 105850T)) and terrestrial bacteria (E. coli (CIP 54.8) and Micrococcus luteus (CIP A270)). The MIC values against A. salmonicida, V. anguillarum, S. baltica, E. coli and M. luteus were as follows: 27, 33, >50, 35 and 47 μM, respectively, for tiahuramide A; 9.4, 8.5, 22, 12 and 29 μM, respectively, for tiahuramide B; and 6.7, 7.4, 16, 14 and 17 μM, respectively, for tiahuramide C. As evidenced by the MIC values, tiahuramide C (20) exhibited the greatest antibacterial potency followed by tiahuramide B (19), whereas tiahuramide A (18) was the least active among this series of compounds ^[19].

Figure 6. Chemical structures of compounds 18–20.

Table 2. Compounds with reported antibacterial activities.

Compound	Source Organism	Collection Site	Targeted Bacteria	MIC/Inhibition Zone/IC₅₀	Reference
17
)
Lyngbya
sp.
Germany (Culture)
B. subtilis
SBUG 14

E. coli
ATCC 11229

E. coli
SBUG 13

P. aeruginosa	ATCC 27853		S. marcescens SBUG 9	18 mm at 25 μg/disk 18 mm at 100 μg/disk 15 mm at 100 μg/disk 8 mm at 100 μg/disk 8 mm at 200 μg/disk	^[18]
Tiahuramide A (18)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 27, 33, >50, 35 and 47 μM, respectively	^[19]
Tiahuramide B (19)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 9.4, 8.5, 22, 12 and 29 μM, respectively	^[19]
Tiahuramide C (20)	L. majuscula	French Polynesia	A. salmonicida (CIP 103209T strain), V. anguillarum (CIP 63.36T), S. baltica (CIP 105850T), E. coli (CIP 54.8) and M. luteus (CIP A270)	MIC = 6.7, 7.4, 16, 14 and 17 μM, respectively	^[19]

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