Observation of color change and UV-Vis spectroscopy
The successful synthesis of AgNPs using of
Aloe barbadensis extracts were confirmed by color changes and spectroscopic analysis. After stirring, the color of the mixture of AgNO
3 and plant extracts changed from pale yellow to reddish - brown in 15 min, revealing the conversion of ionic silver (Ag
+) to metallic silver (Ag) and then into colloidal particles (AgNPs). Ag nanoparticles were concentrated and purified by centrifugal ultrafiltration and then rinsed and dried. Formation of Ag nanoparticles was indicated by theappearance of signature brown colour of the solution (Fig 1). This was the first indication of the efficient synthesis of AgNPs. This observation is consistent with the previous reporting that silver ions are reduced in the presence of plant extracts due to the reducing properties of secondary metabolites (polyphenols, sterols, alkaloids, terpenoids, flavonoids, proteins,
etc.).
The mechanism of Ag
+ reduction reaction to silver nanoparticles is as follow:
4Ag+ + C6H5O7Na3 + 2H2O → 4Ag0 + C6H5O7H3 + 3Na+ + H+ + O2 ↑
In this reaction, the presence of
Aloe barbadensis extract played a role by acting as a stabilizing agent while controlling the silver nanoparticles. Results of electron microscopy analysis showed that silver nanoparticles had quasi-spherical shapes in size range of 5 nm-40 nm (Fig 2).
The wavelength of plant extract nanoparticles was measured using UV-visible Spectroscopy to ascertain the formation of AgNPs. The presence of a strong absorbance peak at about 415 nm clearly indicated the formation of AgNPs due to the surface plasmon resonance (SPR) electrons phenomena present on the nanoparticle surface (Fig 3).
Morphology of isolated pathogenic fungi
Leaf blight areas commonly appear at the edge of the leaf veins, or they might spread over the leaf surface (Fig 4). Leaf blight pathogenic microorganisms were isolated on the PDA. Pathogenic fungi formed fast-growing, thick and smooth white mycelia colonies on PDA plates with black spores at the center. Fungal colonies were yellow, as seen from the bottom view (Fig 5A, 5B). Microscopic morphology of isolated fungi showed that this fungus was cellular with branched hyphae. It produced circular spores attached to the end of hyphae (Fig 6A, 6B). The colony morphology and microscopic morphology of the isolated fungi were similar to those of
M. indicus, which cause rare infections in humans and animals (
Maboni, Krimer et al., 2019). Morphology of the fungal colonies was also similar to
M. indicus isolated from diseased
Ixora coccinea (
Banerjee, Mandal et al., 2018).
Infection of isolated pathogenic fungi on healthy lotus
Healthy 7 -week-old lotus leaves were artificially wounded and infected with isolated fungi. After five days, the appearance of leaf blight was observed. The wound spread, turning reddish-brown and the leaf surface became necrotic (Fig 7). The appearance of leaf blight symptoms on the lotus reconfirmed that isolated fungus was the pathogenic cause of this disease on a lotus.
Identification of isolated mold by 28S rRNA sequencing
DNA extraction, PCR for 28S sequencing, analyzed by Basic Local Alignment Search Tool (BLAST) and compared to sequences of those of the NCBI. Those sequences noted were 100% similar to sequence JE05849, which included 614 nucleotides (score 1134) of
M. indicus. This finding showed that the isolated pathogenic fungus was
M. indicus. This was the first time
M. indicus was reported to cause the leaf blight infection on the lotus (Fig 8).
In vitro antifungal activity and MIC of nano-silver against M. indicus
People usually use synthetic chemicals to treat diseases on plants and crops. However, this process has led to contamination by harmful residue chemicals of produce. On the other hand, nano-silver is being used to treat deadly diseases on these plants in recent times, which has led to higher yields with less harm to human health. Thus, the choice to treat fungi infections on the edible lotus is a safer solution for leaf blight.
Serial dilutions prepared from nano-silver 40 ppm PDA plates against
M. indicus were observed (Fig 9). It was found that increasing silver nano concentration led to more significant and more reliable inhibition of
M. indicus. From a concentration of 1 ppm to 10 ppm, the development of
M. indicus has slightly reduced this growth compared to the control plate. However, concentrations of nano-silver higher than 15 ppm significantly inhibited
M. indices’ growth. Impressively,
M. indicus colonies were tiny spots at concentration 25 ppm and they disappeared at the concentration of 30 ppm. This finding showed that 30 ppm was an
in vitro MIC of nano-silver for its inhibition when applied to
M. indicus.
In vivo antifungal activity of nano-silver
In vitro MIC of nano-silver - 30 ppm - was applied to treat leaf blight
in vivo on artificially induced leaf blight of lotus leaves (Fig 10). Wounds with a size of 1 cm × 1 cm were made on lotus leaves, followed by spraying of
M. indicus liquid culture directly on newly wounded areas. The first treatment was 8 hours after infection. Pathogenic symptoms appeared after 8 hours, leading to a spreading of white areas around the initial wound. Next, the 30 ppm of silver nano was sprayed on diseased leaves. Amazingly, the diseased parts did not expand for 24 hours and remained unchanged until five days later. However, after five days, the diseased areas developed necrosis and turned reddish-brown (Fig 10).
On the other hand, the second treatment was carried out 24 hours after developing an
M. indicus infection. Leaf blight areas (white areas with purple halos) expanded widely on infected leaves after 24 hours of infection. Then this sample was sprayed with 30 ppm nano-silver. The same phenomenon in the previous sample was observed: the leaf blight area stopped expanding and only became necrotic after five days. These impressive inhibitions of leaf blight demonstrated that 30 ppm of nano-silver could inhibit the growth of the pathogenic fungi
M. indicus on lotus leaves.
Mycoleptodiscus indicus is widely known to cause pathogens on many plant parts, causing leaf blight and twig dieback on Ixora coccinea (
Ostazeski, 1967;
Banerjee et al., 2018). On Zamia,
M. indicus caused leaf necrosis with reddish-brown death spots on leaves (
El-Gholl and Alfieri, 1991) similar to the necrotic parts on lotus leaves in this current study. Moreover,
M. indicus is responsible for some diseases on a wide variety of living creatures such as an orchid tree in Malaysia, passion flowers in China, Grape, Ivy,
etc.); animals (cats, dogs in the US,
etc.) and humans
(Maboni et al., 2019). In 2019, an 8-month-old immunocompetent cat in Georgia, USA, was diagnosed with an infection on the front knee with
M. indicus (
Maboni et al., 2019). It also caused several dermal excoriations in an 8-year-old immunodeficient dog
(Metry et al., 2010).
Several cases of
M. indicus infection in humans have been reported. In patients with compromised immune systems, the presence of
M. indicus can lead to this opportunistic pathogenic microorganism causing an infection. For example, it was observed to cause a proliferation of skin nodules with a sporotrichosis lymphangitic distribution in a 51-year-old patient who had just undergone liver transplantation and incidentally suffered from a concurrent infection of HIV and HCV. More seriously, this pathogenic fungus invaded his veins and caused an angioinvasion when performed a histologic analysis
(Garrison et al., 2008). In 2010, a 54 year old Canadian man was reported to have septic arthritis of his left knee caused by an infection with
M. indicus after his three-week vacation to Costa Rica (
Dewar and Sigler, 2010). In addition, a leg infection with
M. indicus was found in a 72-year-old gardener who was also immunodeficient and demonstrated the presence of granulomatosis
(Padhye et al., 1995). In 2012, a glioblastoma multiforme patient was also reported to be infected with necrotizing lesions of his leg caused by
M. indicus (
Koo et al., 2012). These cases were diagnosed with the awareness of the possibility of
M. indicus infections in immunodeficient patients who proved to be compatible hosts of this fungus.
Several cases of
M. indicus infection in humans have been reported. In patients with compromised immune systems, the presence of
M. indicus can lead to this opportunistic pathogenic microorganism causing an infection. For example, it was observed to cause a proliferation of skin nodules with a sporotrichosis lymphangitic distribution in a 51-year-old patient who had just undergone liver transplantation and incidentally suffered from a concurrent infection of HIV and HCV. More seriously, this pathogenic fungus invaded his veins and caused an angioinvasion when performed a histologic analysis
(Garrison et al., 2008). In 2010, a 54-year-old Canadian man reported septic arthritis of his left knee caused by an infection with
M. indicus after his three-week vacation to Costa Rica (
Dewar and Sigler, 2010). In addition, a leg infection with
M. indicus was found in a 72-year-old gardener who was also immunodeficient and demonstrated the presence of granulomatosis
(Padhye et al., 1995). In 2012, a glioblastoma multiforme patient was also infected with necrotizing lesions of his leg caused by
M. indicus (
Koo et al., 2012). These cases were diagnosed with the awareness of the possibility of
M. indicus infections in immunodeficient patients who proved to be compatible hosts of this fungus.