Some Observations on the Growth of Aspergillus niger from Spore Inoculum P. S. KRISHNAN, VIOLOT BAJAJ, AND S. P. DAMLE Division of Biochemistry, National Chemical Laboratory, Poona, India Received for publication May 4, 1954 One of the uncertainties in mold culture is the nature of the spore inoculum. Spores are harvested from agar slants by one of two methods, namely, either by flood- ing the slants with water, or dry, without allowing water to come in contact with the slants. The two types of spore inocula may be expected to give rise to different growth performances in a synthetic medium, since the inoculum prepared by flooding the slants with water would contain material leached out from the mycelium and the agar slant. It was also thought likely that simple washing of the spores with water might yield an inoculum giving re- producible growth performances. Experiments reported in a recent communication from this laboratory (Bajaj, Damle, and Krishnan, 1954) showed that when the spores of Aspergillus niger are collected from synthetic liquid cultures or from agar slants by flooding with water, considerable amounts of inorganic and esterified phosphates pass into solution. McCallan and Wilcoxon (1936), investigating the fungicidal action of Bordeaux mixture, found that malic acid and some amino acids are present in aqueous extracts of spores. The work of Shu and Johnson (1947) indicates that some trace elements can be removed from the conidia by the simple process of washing with water. A number of authors (Mandels and Norton, 1948) have used washed mold spores for biological studies and have reported a high percentage of germination. Perlman (1951) used washed spores of Memnoniella and Aspergillus for inoculating synthetic media and found that the washed spore sus- pensions could be stored in the cold and used over a period of weeks. During the course of experiments designed to study the phosphate metabolism of molds in surface and sub- merged cultures (Krishnan and Bajaj, 1953a, b; Bajaj, Damle, and Krishnan, 1954), the authors have re- peatedly found that the technique of collection of the spores, namely by flooding the slants with water or by removing them dry, affects the cultural characteristics of A. niger and also that the cultures inoculated with washed spores differ from those inoculated with the unwashed spores. Furthermore, the addition of an aqueous extract of potato-dextrose-agar medium has a pronounced influence on the gernination of the spores. An account of these observations is recorded in the following pages. MATERIALS AND METHODS A. niger NRRL 67 was used in all the experiments. Unless otherwise stated potato-dextrose-agar (PDA) served as the sporulation medium and was always made up with distilled water. This medium was dispensed into either 25- x 150-mm test tubes, or litre Roux bottles. A stock soil culture was subcultured thrice and the spores from the last used for inoculating the fer- mentation medium. The spores were harvested by two different techniques. In one, the spores were removed by the suction method of McCallan and Wilcoxon (1936); in the other, the agar slants were flooded with water and the spores dislodged by gentle scraping. The spore count was adjusted to 4 to 7 million conidia per ml. A portion of the spore suspension was set aside for the inoculation of control cultures. The major bulk of the spore suspension was measured, filtered by gravity through Whatman filter paper No. 1 and washed twice on the funnel with distilled water. The entire operation of filtration and washing took 15 to 30 minutes. The residual spore material was resus- pended in water and made up to the original volume. The two supplementary washings of the spores were rejected in some experiments; in others, they were added to the main filtrate and the final volume made up to twice the original volume. The fermentations were carried out under surface and submerged condi- tions, using 250-ml Erlenmeyer flasks for the former and 250- or 500-ml flasks for the latter. Submersion was effected by shaking either in a reciprocal shaker with an amplitude of 1 in at the rate of 127 strokes per minute, or in a rotary shaker describing a circle 3 in in diameter at a speed of 156 rpm. Two-ml aliquots of the spore suspension were used for inoculation of 50 ml portions of Currie's medium made up with 12 per cent dextrose and with the pH adjusted to 1.9. The cultures were divided into the following groups: 1. Control cultures inoculated with the unwashed spores. 2. The rest of the flasks, after prior supplementation as indicated below, inoculated with the washed spores: a) Washed spore cultures, receiving only water. b) Supplemented cultures, receiving 2 ml of the spore filtrate when only the main solution was used, but 4 ml when the two supple- mentary washings also were combined. 303 on April 10, 2019 by guest http://aem.asm.org/ Downloaded from
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Some Observations on the Growth of Aspergillus nigerfrom Spore Inoculum
P. S. KRISHNAN, VIOLOT BAJAJ, AND S. P. DAMLE
Division of Biochemistry, National Chemical Laboratory, Poona, India
Received for publication May 4, 1954
One of the uncertainties in mold culture is the natureof the spore inoculum. Spores are harvested from agarslants by one of two methods, namely, either by flood-ing the slants with water, or dry, without allowing waterto come in contact with the slants. The two types ofspore inocula may be expected to give rise to differentgrowth performances in a synthetic medium, since theinoculum prepared by flooding the slants with waterwould contain material leached out from the myceliumand the agar slant.
It was also thought likely that simple washing of thespores with water might yield an inoculum giving re-producible growth performances. Experiments reportedin a recent communication from this laboratory (Bajaj,Damle, and Krishnan, 1954) showed that when thespores of Aspergillus niger are collected from syntheticliquid cultures or from agar slants by flooding withwater, considerable amounts of inorganic and esterifiedphosphates pass into solution. McCallan and Wilcoxon(1936), investigating the fungicidal action of Bordeauxmixture, found that malic acid and some amino acidsare present in aqueous extracts of spores. The work ofShu and Johnson (1947) indicates that some traceelements can be removed from the conidia by the simpleprocess of washing with water. A number of authors(Mandels and Norton, 1948) have used washed moldspores for biological studies and have reported a highpercentage of germination. Perlman (1951) used washedspores of Memnoniella and Aspergillus for inoculatingsynthetic media and found that the washed spore sus-pensions could be stored in the cold and used over aperiod of weeks.During the course of experiments designed to study
the phosphate metabolism of molds in surface and sub-merged cultures (Krishnan and Bajaj, 1953a, b; Bajaj,Damle, and Krishnan, 1954), the authors have re-
peatedly found that the technique of collection of thespores, namely by flooding the slants with water or byremoving them dry, affects the cultural characteristicsof A. niger and also that the cultures inoculated withwashed spores differ from those inoculated with theunwashed spores. Furthermore, the addition of an
aqueous extract of potato-dextrose-agar medium has a
pronounced influence on the gernination of the spores.An account of these observations is recorded in thefollowing pages.
MATERIALS AND METHODSA. niger NRRL 67 was used in all the experiments.
Unless otherwise stated potato-dextrose-agar (PDA)served as the sporulation medium and was always madeup with distilled water. This medium was dispensedinto either 25- x 150-mm test tubes, or litre Rouxbottles. A stock soil culture was subcultured thrice andthe spores from the last used for inoculating the fer-mentation medium. The spores were harvested by twodifferent techniques. In one, the spores were removedby the suction method of McCallan and Wilcoxon(1936); in the other, the agar slants were flooded withwater and the spores dislodged by gentle scraping.The spore count was adjusted to 4 to 7 million conidiaper ml. A portion of the spore suspension was setaside for the inoculation of control cultures. The majorbulk of the spore suspension was measured, filtered bygravity through Whatman filter paper No. 1 andwashed twice on the funnel with distilled water. Theentire operation of filtration and washing took 15 to30 minutes. The residual spore material was resus-pended in water and made up to the original volume.The two supplementary washings of the spores wererejected in some experiments; in others, they wereadded to the main filtrate and the final volume madeup to twice the original volume. The fermentationswere carried out under surface and submerged condi-tions, using 250-ml Erlenmeyer flasks for the formerand 250- or 500-ml flasks for the latter. Submersionwas effected by shaking either in a reciprocal shakerwith an amplitude of 1 in at the rate of 127 strokes perminute, or in a rotary shaker describing a circle 3 inin diameter at a speed of 156 rpm. Two-ml aliquots ofthe spore suspension were used for inoculation of 50ml portions of Currie's medium made up with 12 percent dextrose and with the pH adjusted to 1.9.The cultures were divided into the following groups:1. Control cultures inoculated with the unwashed
spores.2. The rest of the flasks, after prior supplementation
as indicated below, inoculated with the washed spores:a) Washed spore cultures, receiving only water.b) Supplemented cultures, receiving 2 ml of the
spore filtrate when only the main solutionwas used, but 4 ml when the two supple-mentary washings also were combined.
Days of MyceliumIncu- Utilization plusbation, Acidity of Glucose Spores,No. Dry Wt.*
4
7
12
15
4
7
12
15
4
7
ml N/10NaOH
35431870
326188254304
564390390688
603778501885
38343251
233250219261
416343564
424436424681
26182947
251202273211
per cent
8.79.17.010.1
29.613.018.826.2
52.239.239.260.0
55.668.250.069.7
3.64.94.25.9
24.121.121.425.2
38.837.451.4
43.645.942.257.6
7.94.610.514.0
18.215.221.017.4
84711388231092
* Analysis of mycelium plus spores showed an average of10.35 pg phosphorus and 45.4 ug nitrogen per mg dry weight.
c) PDA cultures, receiving 2 ml of an extract ofthe PDA medium. This extract was preparedby flooding the uninoculated test tube slants
with 5 ml portions of water and drainingafter 15 to 20 seconds.
The volumes were equalized by adding the requisiteamount of distilled water to the cultures. All incuba-tions were carrried out in weak diffused light at roomtemperature (26-28 C), which, during the course of anyone complete series of experiments was held constantwithin 40.5 C. Further details of the culture of theorganism, and of the techniques of assay of the my-celium and of the metabolism fluid, have been reportedelsewhere (Krishnan and Bajaj, 1953a, b). No attemptwas made to separate the spores from the mycelia;the whole mass was dried to constant weight, powderedand sampled. The total nitrogen contents of the my-celia were also determined, being a better indicationof protoplasmic synthesis than dry weight yields. Thedried material was digested with H2S04 aided by H202and the same digest used for phosphate and nitrogenestimations (Damle and Krishnan, 1954).
EXPERIMENTAL RESULTSSurface Culture with Spores Collected by Flooding
Sporulation on the agar slants commenced by aboutthe 2nd day after subculturing. At the end of 4, 6 and9 days the spores were collected by flooding the slantswith water and gentle scraping with a blunt inoculatingneedle. The two supplementary washings were re-jected, the main filtrate only being used in the supple-mented cultures. These filtrates had pH values of 3.81,3.67 and 3.52 on the 4th, 6th and 9th days respec-tively. The analytical data for glucose utilization,acid production, dry weight yields of mat per 100 mlculture solution, and the total phosphate and nitrogencontents per mg of the dried mycelia are recorded intable 1.
It is apparent from the above table that the mycelialyield, glucose utilization and acid production were thehighest in the PDA cultures. The initially formed my-celium in these cultures had a lower nitrogen contentper mg dry weight than the mycelium from the others,but with older cultures such a difference was not ap-parent. The analytical data for total phosphate con-tent per mg dry weight of the mycelia did not show anysystematic variation between the different treatments.There were significant differences in the morphology
of the various cultures. In the control cultures a thinadherent mycelium was formed on about the 3rd day;sporulation commenced within 5 days and was profusewith further incubation. In the washed spore cultures,however, the mycelium was formed in patches andnever joined up during the entire incubation period;also, sporulation was sparse. Supplementation withspore washing induced better, though discontinuousmycelium formation and heavier sporulation thanwere obtained in the washed spore cultures. In thePDA cultures using spores from 4- and 6-day-old agar
slants a well formed mycelium was obtained and sporu-lation was profuse.
Surface Cultures with Spores Collected by the SuctionTechnique
The design of this experiment was the same as thatof the previous one, the only difference being that thetwo supplementary spore washings were combinedwith the main filtrate used in the supplemented cul-tures. The solutions had pH values in the range 5.9 to6.1, which was higher than that of the spore filtratesfrom the flooded spores. The analytical data calculatedon the basis of 100 ml culture solution are recorded intable 2.The data in table 2 show marked differences in the
dry weight yields of the mycelia, glucose utilization,and acid production in the various cultures on the5th day of the incubation. The control and the supple-mented cultures resulted in low yields of mycelia, thewashed spore culture in higher yield, and the PDAculture in the highest yield. The glucose utilization andacid production, also, were the highest in the PDA cul-tures. With further incubation these differences tendedto level out in the various cultures. The data for totalphosphate and nitrogen contents per mg dry weight ofthe mycelia did not show any significant variation dueto the different treatments.The washed spore cuxltures grew in patches and re-
sembled more or less the corresponding cultures ob-tained with spores collected by the flooding technique.However, the control cultures behaved quite differ-ently; the germination of the spores was delayed andthe mycelial formation was very poor even on the 5thday; sporulation, when it set in, was less intense.The supplemented cultures also showed inhibitedgrowth. The PDA cultures, as before, yielded a wellformed mycelium.
Surface Culture on Zn-Supplemented Medium withSpores Collected Dry
Spores from 6-day-old agar slants were used in theexperiments. The fermentation medium contained inaddition to Currie's salts, Zn in a final concentrationof M per 100,000, added in the forn of B.D.H. AnalaRZnSO4-7H20. Besides the four treatments describedearlier a fifth set of cultures was simultaneously run,which was inoculated with the unwashed spores afteradding PDA extract. These served as control plusPDA cultures. The analytical data for the Zn supple-mented, as well as for the control basal medium, cal-culated on the basis of 100 ml culture solution arerecorded in table 3.The data for the 3-day-old culture show that the
control cultures grew very poorly, that myceliumformation was increased on washing the spores, andthat on supplementing the washed spores with the
TABLE 2. Surface culture of Aspergillus niger inoculated withspores of three different ages, collected by suction
4 days agar slantsControl .................Washed.................Supplemented..........P.D.A..................
Control ................Washed.................Supplemented..........P.D.A..................
Control .................Washed.................Supplemented..........P.D.A..................
Control .................Washed.................Supplemented..........P.D.A..................
6 days agar slantsControl .................Washed.................Supplemented..........P.D.A..................
Control .................Washed.................Supplemented..........P.D.A..................
Control .................Washed.................Supplemented..........P.D.A..................
Control .................Washed.................Supplemented..........P.D.A..................
9 days agar slantsControl .................Washed.................Supplemented..........P.D.A..................
TABLE 3. Surface culture of Aspergillus niger in Currie'smedium, with and without Zn, using spores collected by
suction from 6 day slants
Days of With Zn, Without ZnIncubation Dry Wt. Dry Wt.
no mg mg
Control plus P.D.A.......... 3 446 242Control . .65 71Washed . .245 159Supplemented ............... 93 57P.D.A . .392 266
Control plus PDA.......... 5 1023 468Control . .619 288Washed . .748 362Supplemented .............. 539 232P.D.A . .920 500
spore washing, the growth was again retarded. Theaddition of PDA extract either to the washed or theunwashed spores considerably increased the dry weightyields of mycelium. Figure 1 illustrates the morpho-logical appearances of the various cultures on the fourthday in Zn-supplemented medium. On the fifth day ofgrowth, the above differences still persisted but weremuch less pronounced. The presence of Zn in the me-dium gave rise to increased mycelial formation andsuppressed sporulation as compared to the controlbasal medium.
Submerged Culture with Spores Collected Dry and byFlooding
The agar slants used for the collection of spores inthese experiments were 6 days old. The fermentationwas allowed to proceed for onlv 3 to 5 days, since inthe surface cultures the maximum difference between
the various treatments was observed in the initialstages. The depositation of conidia on the sides of theflasks and the subsequent surface growth of mycelium,referred to by Camici, Sermonti, and Chain (1952)were considerably minimized by removing the flasksand shaking at frequent intervals and by rotating theflasks against their own axis while the shaker was inmotion. As a rule, agitation of 500-ml Erlenmeyerflasks containing 75 ml medium in the reciprocatoryshaker yielded the filamentous type of mycelial forma-tion, whereas agitation in the rotary shaker gave riseto a predominantly pellet type mycelium. In the rotaryshaker the PDA cultures consistently showed the small-est pellet formation, somewhat larger pellets werefound in the washed spore cultures, and by far thelargest in the supplemented and the control cultures.It may be recalled that Moyer (1953) recommendedthe addition of about 0.45 per cent agar to shake cul-tures to reduce clumping in the preparation of vegeta-tive inoculum. Results obtained in typical experimentsare recorded in table 4, the data being represented onthe basis of 100 ml culture solution.The data with the reciprocatory shaker are difficult
to interpret, but with the rotary shaker the PDA cul-tures resulted in the highest dry weight yield of mycelialmaterial on the third day. Also, as was observed in thesurface cultures, washing of the spores collected dryled to better mycelial formation than that observed inthe corresponding control cultures. Similarly, the vari-ous treatments of the spores gave rise to mycelialmaterial of fairly constant composition with respect tototal phosphorus and nitrogen. It may also be pointedout that in submerged cultures the acid productionwas much lower than in surface cultures using the samemedium.
FIG: 1. Effect of washing the spores and of addition of an extract of PDA medium on the growth of Aspergillus niger NRRL 67in Currie's medium. Growth 4 days. Left to right: Control plus PDA; PDA; Washed; Supplemented; Control.
The following surface cultures were started using un-
washed suction collected spores:
1. Control cultures, with water added to equalizevolumes.
2. PDA cultures, which received the usual extractof the solidified sporulation medium.
3. Ash cultures, which received an aqueous solutionof the ash obtained from the above extract.
4. Agar cultures, which received an extract fromslants prepared with agar alone, without added potato.The ash cultures as well as the control cultures grew
in patches; the cultures with an extract of agar aloneshowed much better mycelial formation, but by far thebest growth was observed in the cultures which re-ceived an extract of the whole PDA medium. The dryweight yields of mycelia calculated on the basis of 100ml culture solution obtained in a typical experimentare recorded in table 5.The data for the mycelial yields in table 5 indicate
that ash by itself effected a certain stimulation, thatan extract of agar alone increased mycelial formationsignificantly, and that the highest yields were obtainedon supplementing with an extract of the whole medium.The agar effect is therefore a complicated factor, de-rived in part from the inorganic constituents and to agreat extent from the organic constituents of agar andof potato. Whether the stimulation of the spore germi-nation by the organic constituents of PDA is due toa surface effect or/and a growth factor can be decidedonly by further experimentation.
DISCUSSIONThe experimental data recorded in the above pages
indicate the following:The spores of Aspergillus niger NRRL 67 contain
some material inhibitory for surface and, to a less ex-tent, also for submerged growth in Currie's syntheticmedium. This inhibition is prominent in the culturesinoculated with spores collected by suction, but is com-pletely suppressed in cultures with spores collected byflooding.
This inhibitory material can be washed out withwater, so that the washed, suction collected sporesgrow better than the unwashed spores. On supplemen-tation of the washed spore cultures with the sporewashing the growth is again retarded. The inhibition ingrowth is most marked in the initial stages of mycelialformation and tends to disappear with progress in in-cubation of the cultures.
Quite independent of the inhibitory factor presentin, or associated with the spores, is a stimulatory factorpresent in the PDA medium used for sporulation. Onallowing water to remain in' contact with the uninocu-lated slants for about 15 seconds, enough of this activeprinciple is leached out to give a remarkable spurt inthe initial mycelial formation in the shake (rotary) andmuch more so in the surface cultures. This is especiallyprominent in the case of the spores collected by suction,where the inhibition in growth normally observed withthe unwashed spores is totally suppressed by the addi-
tion of an extract of the PDA medium. A spore inoculumprepared by flooding slants with water would contain acertain, though variable, amount of this active factor.With such an inoculum the effect of the inhibitory ma-terial normally associated with the spores is suppressedand the net effect is a fair growth of the mycelium. Thegrowth stimulating effect of the extract of PDA is mostprominent in the early stage of mycelial formation;with further incubation the differences between thePDA and the other cultures wear out.The experiments of Robbins (1939) showed that the
incorporation of agar into an otherwise complete syn-thetic liquid medium resulted in the stimulation ofmycelial growth and gamete production in Phycomycesblakesleanus. Potato and some other naturally occurringmaterial were likewise found to have a growth pro-moting effect on the organism. The accelerated growthresponse of A. niger on the addition of an extract ofPDA medium observed in the present series of investi-gations is probably analogous to the observations ofRobbins on the effect of agar on the culture of Phyco-myces. It is obvious that this agar factor has to bereckoned with in the normal process of inoculation withspores collected by flooding. Varying amounts of theactive material will be removed depending upon thetype and the amount of agar and potato used in com-pounding the medium, the period for which water isallowed to remain in contact with the slant, the even-ness with which the mycelium spreads over the agarsurface, and also the manner in which the spores aredislodged, namely by gentle scraping with a blunt rod,by shaking with glass beads (Dirkx, 1952), or sand(Moyer, 1953), and also the extent to which the activefactor has been depleted from the slants by the growingorganism.
It has to be borne in mind that when water is addedto the slants, some of the material elaborated by thegrowing organism, and secreted into the medium suchas organic acids may get dissolved out. During thebrief period that the water remains in contact with theslants it can also solubilize material from any autolyzedcells of the mycelium and any cells ruptured during theprocess of scraping. Such material may exert an in-fluence on the growth of the culture. In the presentseries of experiments no attempt was made to studythis factor separately.
A. niger NRRL 67 is known to convert sugar pri-marily into citric acid under the conditions of pH andsugar concentration used in the present experiments.No attempt was made to estimate the proportion of thetotal acidity which can be accounted for by citric acid.The data for total acid production given in tables 1and 2 show that inocula prepared by the flooding tech-nique give rise to higher acidity, the highest yield beingfrom cultures containing PDA extract and inoculatedwith spores from 4-day-old agar slants.
ACKNOWLEDGMENTTwo of us (V. B. and S. P. D.) are indebted to the
Council of Scientific and Industrial Research and to theDirector, National Chemical Laboratory, for permissionto work in this Laboratory.
SUMMARYSpores of Aspergillus niger NRRL 67 contain a ma-
terial inhibitory for surface or submerged growth ofthe culture in Currie's synthetic medium. The inhibitorymaterial can be removed from the spores by washingwith water. When the water washings are added backto washed spores, growth is inhibited.The potato-dextrose-agar medium used for sporu-
lation of A. niger contains a stimulatory factor forgrowth. Water extracts of the medium are activein stimulating growth in both surface and submergedcultures. Acid production by A. niger is greater in cul-tures containing water extract from PDA agar than incultures containing no supplement.
REFERENCESBAJAJ, V., DAMLE, S. P., AND KRISHNAN, P. S. 1954 Phos-
phate metabolism of mold spores. 1. Phosphate uptakeby the spores of Aspergillus niger. Arch. Biochem. andBiophys., 50, 451-460.
CAMICI, L., SERMONTI, G., AND CHAIN, E. B. 1952 Obser-vations on Penicillium chrysogenum in submerged culture.1. Mycelial growth and autolysis. Bull. World HealthOrganization, 6, 265-276.
DAMLE, S. P., AND KRISHNAN, P. S. 1954 One step digestionprocedure for the estimation of total phosphorus andnitrogen in mold tissue. Anal. Chim. Acta., 2, 225-228.
DIRKX, J. 1952 L'acide ribonucleique des moisissures. 1.Une technique rapide de dosage. Biochim. et Biophys.Acta, 8, 194-201.
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KRISHNAN, P. S., AND BAJAJ, V. 1953b The polyphos-phatases of Aspergillus niger. 2. Metabolic role of thepolyphosphatases. Arch. Biochem. and Biophys., 47,174-184.
MANDELS, G. R., AND NORTON, A. B. 1948 Studies on thephysiology of spores of the cellulolytic fungus Myro-thecium verrucaria. Research report from the Quarter-master General Laboratories, Microbiology series No. 11,1-50.
MCCALLAN, S. E. A., AND WILCOXON, F. 1936 The action offungus spores on Bordeaux mixture. Contrib. BoyceThompson Inst., 8, 151-165.
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PERLMAN, D. 1951 The effects of biologically active agentson fungi at different stages of growth. Am. J. Botany,38, 652-8.
ROBBINS, W. J. 1939 Growth substances in agar. Am. J.Botany, 26, 772-778.
SHU, P., AND JOHNSON, M. J. 1947 Effect of composition ofsporulation medium on citric acid production by Asper-giUlus niger in submerged culture. J. Bacteriol., 54, 161-167.
